Broad band referencing reflectometer

ABSTRACT

A spectroscopy system is provided which is optimized for operation in the VUV region and capable of performing well in the DUV-NIR region. Additionally, the system incorporates an optical module which presents selectable sources and detectors optimized for use in the VUV and DUV-NIR. As well, the optical module provides common delivery and collection optics to enable measurements in both spectral regions to be collected using similar spot properties. The module also provides a means of quickly referencing measured data so as to ensure that highly repeatable results are achieved. The module further provides a controlled environment between the VUV source, sample chamber and VUV detector which acts to limit in a repeatable manner the absorption of VUV photons. The use of broad band data sets which encompass VUV wavelengths, in addition to the DUV-NIR wavelengths enables a greater variety of materials to be meaningfully characterized. Array based detection instrumentation may be exploited to permit the simultaneous collection of larger wavelength regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/517,894 filed on Sep. 8, 2006 which is a continuation of U.S. patentapplication Ser. No. 10/909,126 filed on Jul. 30, 2004, now U.S. Pat.No. 7,126,131, which is a continuation-in-part of (1) U.S. patentapplication Ser. No. 10/669,030 filed on Sep. 23, 2003, now U.S. Pat.No. 7,026,626, which claims priority to Provisional Patent ApplicationNos. 60/440,434, 60/440,435, and 60/440,443 all filed Jan. 16, 2003; (2)U.S. patent application Ser. No. 10/668,642 filed on Sep. 23, 2003, nowU.S. Pat. No. 7,067,818, which claims priority to Provisional PatentApplication Nos. 60/440,434, 60/440,435, and 60/440,443 all filed Jan.16, 2003; and (3) U.S. patent application Ser. No. 10/668,644 filed onSep. 23, 2003, now U.S. Pat. No. 7,394,551, which claims priority toProvisional Patent Application Nos. 60/440,434, 60/440,435, and60/440,443 all filed Jan. 16, 2003; the disclosures of which are eachexpressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of optical metrology. Morespecifically it provides a means by which repeatable reflectancemeasurements may be performed over a broad range of wavelengths thatincludes the vacuum ultraviolet (VUV) (generally wavelengths less than190 nm) and at least one other spectral region.

Optical metrology techniques have long been employed in process controlapplications in the semiconductor manufacturing industry due to theirnon-contact, non-destructive and generally high-throughput nature. Thevast majority of these tools operate in some portion of the spectralregion spanning the deep ultraviolet and near-infrared wavelengths(DUV-NIR generally 200-1000 nm). The push towards thinner layers and theintroduction of new complicated materials has challenged the sensitivityof such instrumentation. As a result, this has necessitated an effort todevelop optical metrology equipment utilizing shorter wavelengths (below200 nm), where greater sensitivity to subtle changes in materialproperties can be realized. One approach to performing opticalmeasurements at shorter wavelengths is described in U.S. applicationSer. No. 10/668,642, which discloses a system and method for a vacuumultraviolet (VUV) reflectometer.

Virtually all optical metrology instruments incorporate some form ofmodeling algorithms to extract meaningful material information from thequantities they initially record. The performance of such algorithmsdepends heavily on the nature of the data sets they are to reduce. Datasets covering a wider range of wavelengths generally provide moreconstraint to fitting algorithms thereby rendering faster convergenceand more accurate results.

The conventional approach to collecting reflectance data over a broadrange of wavelengths covering at least two spectral regions is to employa step and scan technique wherein a single element detector is used inconjunction with a rotating grating monochromator. Often if the range ofwavelengths investigated is large enough, it may be necessary tomanually change out gratings, detectors, optics and sources during theacquisition of a single broad-band data set. This approach is often timeconsuming and not well suited to manufacturing environments like thoseencountered in the semiconductor industry.

Interferometers are widely used in the infrared spectral region tocollect data over a wide range of wavelengths; however, theseinstruments are not commonly employed in the VUV since optical andmechanical tolerances of the instrument scale with wavelength and aredifficult to satisfy in this spectral region.

Wang, in U.S. Patent Application 20030071996, discloses a measurementsystem with separate optimized beam paths. Although this system enablesefficient measurements to be performed over a number of spectralsub-bands, it provides no means of referencing the collected data.Hence, while signal throughput may be high, system repeatability may bequite poor. This is particularly relevant when operating in the VUVsince such wavelengths are highly susceptible to atmospheric changesnecessitating frequent referencing.

The collection of highly repeatable reflectance data in the VUV isperhaps best achieved using a system designed to minimize and/oraltogether eliminate errors introduced by data altering environmentalchanges which may occur between conclusion of a calibration measurementand commencement of a subsequent sample measurement. An example of sucha system is described in U.S. application Ser. No. 10/668,644. Theapplicant has identified that it would be desirable to extend thiscapability in order to facilitate its use in a reflectometer capable ofacquiring data over a broad range of wavelengths including the VUV andat least one other spectral region.

The applicant has further identified that it would be desirable toensure that data sets from each of the spectral regions comprising theentire broad band of wavelengths are collected from the same physicallocation on the sample and with the same spot size. Moreover, it wouldalso be advantageous if such data sets were collected using the sameorientation (i.e. angle of incidence and direction) relative to thesample in order to ensure that similar scattering conditions areencountered.

The applicant has also identified that it would be desirable if saidsystem made use of a serial collection process wherein data from each ofthe spectral regions were collected sequentially to avoid stray lightcomplications, which one would expect if a parallel process wasemployed.

SUMMARY OF THE INVENTION

An objective of the current invention is to provide the semiconductormanufacturing industry with a reliable optical metrology tool that iscapable of characterizing semiconductor devices incorporating thinnerlayers and new complicated materials. Any fitting algorithms employed bya user of the instrument may achieve faster convergence and moreaccurate results by taking full advantage of the higher level ofconstraint afforded by a data set comprised of two or more spectralregions. This instrument will be non-contact and non-destructive andwill make use of broad band reflectance data.

The instrument will be optimized for operation in a first spectralregion and capable of performing well in at least one other spectralregion. A selection of sources and detectors for use in separatespectral regions are incorporated within an optical module in theinstrument that permits their selection. Additionally, this modulecontains common delivery and collection optics to enable measurements inseparate spectral regions to be collected using similar spot properties.Furthermore, the invention employs a serial collection approach wherebydata from separate spectral regions is collected sequentially to avoidstray light complications.

In one embodiment a spectroscopy system is provided which is optimizedfor operation in a first spectral region and capable of performing wellin at least one other. The system is designed such that no movingoptical elements (apart from shutters) are involved in the collection ofdata from the first spectral region. Additionally, the systemincorporates an optical module which presents selectable sources anddetectors optimized for separate spectral regions. As well, the opticalmodule provides common delivery and collection optics to enablemeasurements in separate spectral regions to be collected using similarspot properties. The module also provides a means of quickly referencingmeasured data so as to ensure that highly repeatable results areachieved.

In another embodiment a spectroscopy system is provided which isoptimized for operation in the VUV and capable of performing well in theDUV-NIR. Additionally, the system incorporates an optical module whichpresents selectable sources and detectors optimized for use in the VUVand DUV-NIR. As well, the optical module provides common delivery andcollection optics to enable measurements in both spectral regions to becollected using similar spot properties. The module also provides ameans of quickly referencing measured data so as to ensure that highlyrepeatable results are achieved. The module further provides acontrolled environment between the VUV source, sample chamber and VUVdetector which acts to limit in a repeatable manner the absorption ofVUV photons. The use of broad band data sets which encompass VUVwavelengths, in addition to the DUV-NIR wavelengths enables a greatervariety of materials to be meaningfully characterized. Array baseddetection instrumentation may be exploited to permit the simultaneouscollection of larger wavelength regions.

An objective of the current invention is to provide the semiconductormanufacturing industry with a reliable optical metrology tool that iscapable of characterizing semiconductor devices incorporating thinnerlayers and smaller features. This instrument will take full advantage ofthe heightened levels of sensitivity to small changes in processingconditions that are afforded through the characterization of materialsat and below the VUV wavelength region. This instrument will benon-contact and non-destructive and will make use of referencedreflectometry data.

The design of the instrument is simple and robust rendering it easy tooperate at VUV wavelengths. As a result of its simplicity it is fast,thereby lending it to use in high throughput manufacturing applications.The system is designed such that it is compact and requires littlespace. The instrument offers the advantage of minimizing environmentaleffects so far as to remove the need to sustain a reproduciblecontrolled environment during the course of calibration, referencing andsample measurement. Additionally, it offers a small measurement spotsize such that in-line measurements on product samples are possible andemploy imaging optics such that multiple measurements on a series ofsites within a localized region can be obtained simultaneously.

In one embodiment, a spectroscopy system is provided which operates inthe vacuum ultra-violet spectrum. More particularly, a system utilizingreflectometry techniques in the vacuum ultraviolet spectrum is providedfor use in metrology applications. The system may further include theuse of an array detector in combination with an imaging spectrometer. Inthis manner data for multiple wavelengths may be simultaneouslycollected. Moreover, the multiple wavelengths of data may be collectedsimultaneously for a two dimensional sample area. The system may furtherinclude the use of a fixed diffraction grating and does not require theuse of polarizing elements. To ensure accurate and repeatablemeasurement, the environment of the optical path is controlled. Theoptical path may include a controlled environmental chamber in whichnon-absorbing purge gases are present or in which vacuum evacuationtechniques are utilized. The controlled environment may further includea separate instrument chamber and a separate sample chamber. Thecontrolled environment limits in a repeatable manner the absorption ofVUV photons.

In another embodiment, a spectroscopy system is provided which operatesin the vacuum ultra-violet spectrum. More particularly, a systemutilizing reflectometry techniques in the vacuum ultraviolet spectrum isprovided for use in metrology applications. To ensure accurate andrepeatable measurement, the environment of the optical path iscontrolled to limit absorption effects of gases that may be present inthe optical path. To account for absorption effects that may stilloccur, the length of the optical path is minimized. To further accountfor absorption effects, the reflectance data may be referenced to arelative standard. Referencing is particularly advantageous in the VUVreflectometer due to the low available photon flux and the sensitivityof recorded data to the composition of the gaseous medium contained withthe optical path. Thus, errors that may be introduced by changes in thecontrolled environment may be reduced. In one exemplary embodiment, theVUV reflectometer may utilize a technique in which a beam splitter isutilized to create a sample beam and a reference beam to form the twoarms of a near balanced Mach Zehnder interferometer. In anotherexemplary embodiment, the reference channel may be comprised of aMichelson interferometer.

In still another embodiment, a spectroscopy system is provided whichoperates in the vacuum ultra-violet spectrum. More particularly, asystem utilizing reflectometry techniques in the vacuum ultravioletspectrum is provided for use in metrology applications. To ensureaccurate and repeatable measurement, the environment of the optical pathis controlled to limit absorption effects of gases that may be presentin the optical path. The VUV reflectometer may be utilized to monitor awide range of data in a semiconductor processing environment. Forexample, the techniques may be used for measuring thicknesses, opticalproperties, composition, porosity and roughness of a film or stack offilms. Further, these VUV techniques and apparatus may be used tocharacterize critical dimensions and other features of a device. The VUVreflectometer system may be utilized as a stand alone tool, or therelatively compact nature of the system may be taken advantage of suchthat the system is incorporated into other process tools. Thus, forexample, the VUV techniques described herein may be incorporateddirectly into tools used for deposition, etch, photolithography, etc. sothat in-line measurements, monitoring and control may be advantageouslyobtained.

A further understanding of the nature of the advantages of the presentinvention may be realized following review of the following descriptionsand associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencesnumbers indicate like features. It is to be noted, however, that theaccompanying drawings illustrate only exemplary embodiments of theinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1—Comparison of optical transmission through 100 cm of standardatmosphere versus 100 cm of controlled environment containing 1 PPM ofH2O and O2.

FIG. 2—Schematic representation of a VUV reflectometer.

FIG. 3—Spectral output from Hamamatsu deuterium lamp equipped with anMgF2 window.

FIG. 4—“Solar-blind” broad-band VUV filter from Acton ResearchCorporation.

FIG. 5—Top-down schematic view of VUV reflectometer illustratingincorporation of reference channel.

FIG. 6—Typical off-axis parabolic mirror.

FIG. 7—Broad-band VUV-UV reflective coatings from Acton ResearchCorporation.

FIG. 8—Example of “through-pellicle” measurement using shallow angle(solid line) and large angle (dashed) incidence configurations.

FIG. 9—Use of imaging reflectometer to simultaneously record multiplespectra from different physical locations on patterned sample.

FIG. 10—Schematic view of alternate VUV reflectometer with referencechannel.

FIG. 11—Schematic view of alternate VUV reflectometer with virtually alloptics housed within instrument chamber.

FIG. 11 a—Alternate schematic view of the system of FIG. 11.

FIG. 11 b—Schematic view of a system of FIG. 11 integrated with aprocess tool.

FIG. 12—Typical measurement flow chart.

FIG. 12 a—(split as FIGS. 12A-A and 12A-B)—Exemplary detailedmeasurement flow chart.

FIG. 13—Typical properties associated with VUV beam splittermanufactured by Acton Research Corporation.

FIG. 14—Error plots as a function of concentration differences and pathlength differences.

FIG. 15—Schematic representation of typical reflectance measurement.

FIG. 16—Measured and calculated reflectance spectra from thin Al2O3layer deposited on silicon substrate.

FIG. 17—Optical properties (n and k values) obtained for Al2O3 layerthrough iterative fitting process.

FIG. 18—Reflected signal associated with ultra-thin (5 Å) layer ofresidual photo resist deposited on mask blank. Difference signalcorresponding to a 1 nm increase in layer thickness.

FIG. 19—Reflectance signal from 10 Å, 14 Å, and 18 Å SiON layers onsilicon substrates.

FIG. 20—Reflectance difference signal at 130 nm, 157 nm and 193 nmassociated with increase in film thickness for a 10 Å SiON layer.

FIG. 21—Reflectance signal associated with matrix of 16 Å SiON layerspossessing nitrogen concentrations in the range of 10-15%.

FIG. 22—Reflectance difference signal at 130 nm, 157 nm and 193 nmassociated with increase in nitrogen concentration for a 16 Å SiON layerwith 10% nitrogen.

FIG. 22 a—Different nitrogen doping profiles for a 20 Å SiON film. Inall cases the samples were exposed to the same dose of 1e¹⁵ atoms/cm².

FIG. 22 b—Reflectance difference signals (relative to the uniformlydoped sample) resulting from a variety of non-uniform nitrogendistributions.

FIG. 23—Interaction of incident DUV and VUV photons with typicalsemiconductor stack sample during reflectance measurement.

FIG. 24—Reflectance spectra from SiO₂/SiN/Si samples exhibitingdifferent SiN thicknesses.

FIG. 25—Reflectance spectra from SiO₂/SiN/Si samples exhibitingdifferent SiO₂ layer thicknesses.

FIG. 26—Schematic representation of typical scatterometry measurementillustrating both reflected and diffracted beams.

FIG. 27—Schematic representation of typical outputs obtained throughscatterometry measurements.

FIG. 28—Reflected signal associated with nominal 65 nm line array anddifference signal corresponding to 1 nm increase in nominal 65 nm linewidth.

FIG. 29—Reflected signal associated with line arrays comprised of 63 nm,65 nm and 67 nm lines and spaces.

FIG. 30—Reflected signal associated with line array comprised of 65 nmwide lines and spaces (for nominal line height of 1000 Å). Differencesignal corresponding to 10 Å increase in line height of said structure.

FIG. 31—Schematic representation of broad-band reflectometer withoptical module.

FIG. 32—Broad-band referencing reflectometer covering VUV and DUV-NIRspectral regions.

FIG. 33—Serial measurement flowchart for broad-band referencingreflectometer covering VUV and DUV-NIR spectral regions.

FIG. 34—Broad-band referencing reflectometer covering three spectralregions.

FIG. 35—Serial measurement flowchart for broad-band referencingreflectometer covering three spectral regions.

FIG. 36—Alternate embodiment of broad-band referencing reflectometerusing rotating mirrors and covering VUV and DUV-NIR spectral regions.

FIG. 37—Alternate embodiment of broad-band referencing reflectometercovering three spectral regions.

FIG. 38—Alternate embodiment of broad-band referencing reflectometerwithout flip-in mirrors and covering two spectral regions.

FIG. 39—Serial measurement flowchart for broad-band referencingreflectometer without flip-in mirrors and covering two spectral regions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To enhance the sensitivity of optical metrology equipment forchallenging applications it is desirable to extend the range ofwavelengths over which such measurements are performed. Specifically, itis advantageous to utilize shorter wavelength (higher energy) photonsextending into, and beyond, the region of the electromagnetic spectrumreferred to as the vacuum ultra-violet (VUV). Historically there hasbeen relatively little effort expended on the development of opticalinstrumentation designed to operate at these wavelengths, owing to thefact that VUV (and lower) photons are strongly absorbed in standardatmospheric conditions. Vacuum ultra-violet (VUV) wavelengths aregenerally considered to be wavelengths less than deep ultra-violet (DUV)wavelengths. Thus VUV wavelengths are generally considered to bewavelengths less than about 190 nm. While there is no universal cutofffor the bottom end of the VUV range, some in the field may consider VUVto terminate and an extreme ultra-violet (EUV) range to begin (forexample some may define wavelengths less than 100 nm as EUV). Though theprinciples described herein may be applicable to wavelengths above 100nm, such principles are generally also applicable to wavelengths below100 nm. Thus, as used herein it will be recognized that the term VUV ismeant to indicate wavelengths generally less than about 190 nm howeverVUV is not meant to exclude lower wavelengths. Thus as described hereinVUV is generally meant to encompass wavelengths generally less thanabout 190 nm without a low end wavelength exclusion. Furthermore, lowend VUV may be construed generally as wavelengths below about 140 nm.

Indeed it is generally true that virtually all forms of matter (solids,liquids and gases) exhibit increasingly strong optical absorptioncharacteristics at VUV wavelengths. Ironically it is this same ratherfundamental property of matter which is itself responsible for theincreased sensitivity available to VUV optical metrology techniques.This follows as small changes in process conditions, producingundetectable changes in the optical behavior of materials at longerwavelengths, can induce substantial and easily detectable changes in themeasurable characteristics of such materials at VUV wavelengths.

The fact that VUV photons are strongly absorbed by most forms of matterprecludes the simple extension of, or modification to, conventionallonger wavelength optical metrology equipment in order to facilitateoperation in the VUV. Current day tools are designed to operate understandard atmospheric conditions and typically lack, among other things,the controlled environment required for operation at these shorterwavelengths. VUV radiation is strongly absorbed by both O₂ and H₂Omolecules and hence these species must be maintained at sufficiently lowlevels as to permit transmission of VUV photons through the optical pathof the instrument. To better illustrate this point the opticaltransmission through a 100 cm path length of both standard atmosphere(plot 100) and a controlled environment containing O₂ and H₂Oconcentration levels of 1 PPM (plot 110) are plotted as a function ofphoton wavelength in FIG. 1. As is evident from the figure, thetransmission of photons through standard atmosphere drops precipitouslyat wavelengths shorter than about 200 nm.

Not only are conventional optical instruments intended to function instandard atmospheric conditions they also typically employ an array ofoptical elements and designs which render them unsuitable for VUVoperation. In order to achieve highly repeatable results with areflectometer it is desirable to provide a means by which reflectancedata can be referenced or compared to a relative standard. In thismanner changes in the system that occur between an initial time when thesystem is first calibrated and a later time when a sample measurement isperformed, can be properly accounted for. At longer wavelengths suchchanges are usually dominated by intensity variations in the spectraloutput of the source. When working at VUV wavelengths, however, changesin the environmental conditions (i.e. changes in the concentration ofabsorbing species in the environment of the optical path) can play amuch larger role.

Thus, conventional longer wavelength systems fail to address thesignificant influence that the absorbing environment has on themeasurement process. To ensure that accurate and repeatable reflectancedata is obtained, it is desirable to not only provide a means ofcontrolling the environment containing the optical path, but furthermoreto ensure that the absorption effects which do occur are properly takeninto account during all aspects of the calibration, measurement andreference processes.

Hence, it is desirable to provide an optical metrology tool with acontrolled environment that is designed to operate at and below VUVwavelengths. In addition, in order to ensure that accurate andrepeatable results are obtained, it is desirable that the designincorporate a robust referencing methodology that acts to reduce oraltogether remove errors introduced by changes in the controlledenvironment.

A schematic representation of an optical reflectometer metrology tool200 that depicts one embodiment of the present invention is presented inFIG. 2. As is evident, the source 210, beam conditioning module 220,optics (not shown), spectrometer 230 and detector 240 are containedwithin an environmentally controlled instrument chamber 202. The sample250, additional optics 260, motorized stage 270 (which may include anoptional desorber) are housed in a separate environmentally controlledsample chamber 204 so as to enable the loading and unloading of sampleswithout contaminating the quality of the instrument chamber environment.The instrument and sample chambers are connected via a controllablecoupling mechanism 206 which can permit the transfer of photons, and ifso desired the exchange of gases to occur. For example, couplingmechanism 206 may be optical windows, may be gate valves which open whenan optical transmission path is desired, or may be other mechanisms thatsuitably allow an optical path to be coupled between the two chambers.In this manner an optical path between the instrument and samplechambers is provided. Additionally a processor 290 located outside thecontrolled environment may be used to analyze the measured data. It willbe recognized that processor 290 may be any of a wide variety ofcomputing means that may provide suitable data processing and/or storageof the data collected.

While not explicitly shown in FIG. 2, it is noted that the system couldalso be equipped with a robot and other associated mechanized componentsto aid in the loading and unloading of samples in an automated fashion,thereby further increasing measurement throughput. Further, as is knownin the art load lock chambers may also be utilized in conjunction withthe sample chamber to improve environmental control and increase thesystem throughput for interchanging samples.

In operation light from the source 210 is modified, by way of beamconditioning module 220, and directed and focused via delivery opticsthrough the coupling mechanism windows 206 and onto the sample 250housed in the sample chamber 204. Light reflected from the sampletravels back through the coupling mechanism 206, is captured bycollection optics and focused onto the entrance plane of a spectrometer230. As is discussed in more detail below, the spectrometer 230 may bein one embodiment an imaging spectrometer. This type of spectrometer istypically equipped with some form of multi-element detector 240 (forexample an array detector), such that it is capable of collecting arange of data points simultaneously. The entire optical path of thedevice is maintained within controlled environments which function toremove absorbing species and permit transmission of VUV photons. Thecontrolled environments may be created with purge or vacuum system 280by introducing a non-absorbing purge gas like high purity nitrogen,argon, or helium into the instrument and sample chambers and/or throughevacuation via vacuum, depending on the lowest operating wavelengthdesired. If a high purity purge gas is used, the coupling mechanism 206could be comprised of MgF₂ windows, whereas if the chambers areevacuated then mechanical gate valves could be employed. Otherpotentially suitable window materials include fused silica,fluorine-doped fused silica, quartz, CaF, SrF, BaF, MgF₂, LaF and LiF.It will be recognized that by utilizing a combination of evacuationtechniques and mechanical gate valves, absorption of photons may befurther decreased.

In one embodiment of the invention the VUV source 210 is a long noseprojecting type deuterium (D₂) lamp, such as Model #L7293 manufacturedby Hamamatsu of Japan. Such a source is a broad band VUV source andcombines mature arc lamp technology with a magnesium fluoride (MgF₂)window to provide continuous emission down to about 115 nm (see plot 300of FIG. 3). The window may however be comprised of a variety of VUVmaterials including fused silica, fluorine-doped fused silica, quartz,CaF, SrF, BaF, MgF₂, LaF and LiF. The projecting design of the lampprovides superior directivity thereby enabling efficient coupling of VUVphotons into the reflectometer optical system. D₂ arc lamps arecharacterized by high stability, high brightness and long-life renderingthem well-suited to demanding semiconductor metrology applications.Alternate embodiments of the invention may incorporate a variety ofother VUV sources including, but not limited to, narrow band sources andwindowless discharge sources which could emit photons at wavelengthsdown to and below 115 nm. For example, the windowless source may be adifferentially pumped discharge source. Thus, the systems and techniquesprovided herein are particularly useful for low end VUV (or lower)applications.

Referring again to FIG. 2, the beam conditioner module 220 allows forthe introduction of spatial and/or spectral filtering elements to modifythe properties of the source beam. While this functionality may notgenerally be required, there may arise specific applications where it isdeemed advantageous. Examples could include modifying the spatial ortemporal coherence of the source beam through use of an aperture, orintroduction of a “solar blind” filter to prevent longer wavelengthlight from generating spurious VUV signals through scattering mechanismsthat may occur at the various optical surfaces in the optical beam path.In a particular embodiment of the device the “solar blind” filter is aVUV filter from Acton Research Corporation, typical reflectiveproperties of which are presented in FIG. 4 as shown by plot 400 for theActon Research part number 120-VBB filter and plot 410 for an ActonResearch part number 170-VBB filter.

A more detailed diagram of one embodiment of an optical reflectometermetrology tool 500 is provided in FIG. 5, wherein the optics comprisingthe measurement and reference channels of the device are illustrated inmore detail. Though not shown, it will be recognized that the opticalreflectometer metrology tool may include the components shown in FIG. 2such as the purge or vacuum system 280, processor 290, stage 270, etc.As shown in FIG. 5, a source 510, spectrometer 530, and array detector540 may be provided in an instrumentation chamber 502. A sample chamber504 is coupled to the instrumentation chamber 502 through couplingmechanisms (not shown).

Referring again to FIG. 5 the optical path for a sample measurement willbe described. It is seen that light from the source 510 is collimatedand directed by Mirror 1 towards Beam Splitter 1, where the source beamis split into sample and reference beam components (generally indicatedby beams 508 and 506 respectively). The sample beam 508 is reflectedfrom the beam splitter 1 towards plane Mirror 5, where it is redirectedtowards Mirror 2. Here the light is focused down (into the plane of thefigure) onto the sample 550. The reflected light (out of the plane ofthe figure) from the sample 550 is captured by the collimating optic(Mirror 3) where it is directed through Beam Splitter 2 towards thefocusing optic, Mirror 4. Here the light is then focused onto theentrance plane of the spectrometer 530. During measurement of the sampleShutters 1 and 2 are open while Shutter 3 remains closed.

In one embodiment, the mirror 1, mirror 2, mirror 3 and mirror 4 areoff-axis parabolic reflectors; an example of such is depicted asoff-axis mirror 600 in FIG. 6. These mirrors are preferably polishedusing conventional techniques following their manufacture and thencovered with some form of broad band reflective coating 610 like Al/MgF₂(some manufacturers may implement aluminum and MgF₂ layers directly oneach other on the mirror or alternatively thin layers of other materialsmay be located under or over the aluminum layer). Post polishingimproves the imaging properties of the mirrors by minimizing issuesarising from diamond turning artifacts. The broad band coating 610 istailored to enhance the reflective properties of the mirrors in the VUV.Examples of particularly well-suited coatings for coating 610 areproduced by Acton Research Company. FIG. 7 illustrates reflectance plotsfor coating #1000, #1200, and #1600 produced by Acton ResearchCorporation (plots 700, 710 and 720 respectively). For operation atshorter wavelengths other coatings like elemental iridium may be bettersuited.

While other types of mirrors could also be incorporated into the system,the use of off-axis parabolic reflectors enables reflectance data to beobtained using near-normal incidence illumination/collection yieldingnumerous benefits. These advantages include simplifying the subsequentanalysis of measured data since polarization effects can be neglected,yielding symmetric illumination of measurement regions on the sample,minimizing scattering effects at the sample surface encountered usinglarger angles of incidence and facilitating compact system design, animportant consideration for integrated and/or in-line metrologyapplications. Additionally, certain optical measurements may themselvesbenefit from use of a near-normal configuration. Typical examplesinclude, but are not limited to, dimensional characterization of highaspect ratio features using scatterometry methods and through-pelliclemeasurements of photo masks. For example as shown in FIG. 8 and known inthe art, a semiconductor photolithography mask substrate 800 may have afilm (or films) 810 that is (are) protected by a pellicle 820.Measurements of the through pellicle measurements of the film (films)810 may benefit from the use of a near normal configuration.

Off-axis parabolic mirrors are regularly produced by a variety of opticsmanufactures; they are as such, readily available and relativelyinexpensive. They offer greater degrees of freedom, with respect tointegration and alignment, and do not suffer from astigmatism to thesame extent as toroidal mirrors when used in similar applications.

In a particularly useful embodiment of the invention one or more of theoff-axis parabolic mirrors is designed such that the off-axis angle(denoted as θ in FIG. 6) is equal to 90°. Such an arrangement providesconsiderable flexibility and reduced susceptibility to scattering atshorter wavelengths (a consequence of the smaller incident anglesinvolved). Flexibility follows from the fact that rotation of one suchoptic about an axis parallel to the central ray axis of the optic mapsout a two dimensional pattern (i.e. circle) as opposed to a threedimensional pattern (i.e. cone) for optics possessing other off-axisangles. This specific geometrical configuration offers a number ofsystem enhancement possibilities and advantages.

An example of one such enhancement would be to enable simpleincorporation of multiple sources into the system. Other sources couldbe placed at appropriate locations around an axis perpendicular to thecentral ray axis of the optic. To select another source it would thenonly be necessary to rotate the optic around the axis. Another advantageof such an arrangement would be realized during the initial alignmentphase of the instrument. Using a normal incidence configuration wouldenable simple determination of proper alignment on both the illuminationand collection arms of the tool since they would be working on-axis, inthe sense that they would be focusing onto a surface perpendicular tocentral focusing. This results in better spot definition and hence,better overall imaging performance.

Referring again to FIG. 5, once the light enters the spectrometer 530 itis reflected by a plane mirror 531, collimated by a focusing mirror 532and incident upon a diffraction grating 533. Some portion of the lightdiffracted by the grating is collected by the second focusing mirror 534and focused onto the surface of the VUV sensitive array detector 540. Asis known in the art, the light that is reflected from the diffractiongrating becomes spatially separated by wavelength across the width ofthe detector. It is noted that in this particular embodiment all of theoptics inside the spectrometer have also been coated with broad bandreflective coatings like Al/MgF₂ to increase their efficiencies.Ideally, the spectrometer is an imaging spectrometer that is designed insuch a manner as to provide stigmatic imaging in a large area flat fieldas is the case with the 250 is/sm manufactured by Chromex Instruments(see also U.S. Pat. No. 4,932,768). Such spectrometers typically allow awide range of multiple wavelengths to exit the spectrometersimultaneously for detection by the detector element (as opposed to sometypes of spectrometers which attempt to restrict the exiting light to asingle wavelength). Typically, such spectrometers utilize a fixeddiffraction grating since a moveable diffraction grating is not requiredto generate the data at varying wavelengths. The imaging spectrometermay be utilized in combination with an array detector such that themultiple wavelengths exiting the spectrometer may be spread across thewidth of the array detector. The columns across the width of thedetector are thus presented with light of different wavelengths. Theinternal elements of imaging spectrometers may be designed such that themultiple wavelengths are sufficiently resolved so that the arraydetector may accurately obtain data for various wavelengths.

In addition, it is advantageous if the diffraction gratings are of theholographic ion-etched type so as to minimize stray light resulting fromlight scattering at short wavelengths. Alternate embodiments of theinvention could also incorporate other types of VUV spectrometersincluding non-periodic toroidal grating configurations, like thosemanufactured by Jobin-Yvon of France, Rowland circle configurations,like those manufactured by Resonance Ltd. of Canada, or Echeletteconfigurations like those manufactured by Catalina Scientific Corp. ofthe United States. In addition, the diffraction grating utilized neednot be a movable but rather may be implemented as a fixed diffractiongrating.

While any number of VUV sensitive array detectors could be used with theinvention, it is desirable to use a detector that provides efficientconversion of VUV photons while offering a wide dynamic range.Back-thinned, back-illuminated, uncoated charge coupled devices (CCD's)are particularly well-suited for this application as they offer highsensitivity and avoid losses due to absorption of VUV photons in poly-Sigate regions as encountered by their phosphorus coated front-illuminatedcounterparts. Uncoated devices are generally expected to perform betterover a wide range of wavelengths than those possessing anti-reflectionlayers. Another type of array detector that may be used is a microchannel plate detector coupled to a standard CCD or photodiode array(PDA). One example of a micro channel plate detector suitable for thisapplication is manufactured by Burle Industries, Inc. of the UnitedStates. Alternatively, front-illuminated CCD's or photodiode arrays canindependently be used if they are equipped with a phosphorus coatingthat absorbs short wavelength photons and re-emits longer wavelengthphotons that can then be effectively collected with the devices.

Another aspect of the array detector 540 is that it may be cooled to lowtemperatures (below 0° C.) to reduce dark counts (i.e. thermallygenerated carriers) which mask a measured signal and can adverselyaffect system accuracy in cases where low photon levels prevail. Inorder to cool the detector, it may be necessary to encapsulate it in ahermetically sealed chamber to prevent condensable species fromaccumulating on the device. This is usually accomplished by mounting thedevice in a vacuum chamber sealed with an MgF₂ window to permit VUVphotons to pass. For operation at shorter wavelengths (generally belowabout 115 nm, the transmission cutoff for MgF2) the protective windowcould be removed as the controlled environment would be that of vacuum,rather than a non-absorbing purge gas. A particularly well-suiteddetector (Model # DV420-BN) is manufactured by Andor Technology ofNorthern Ireland. This particular detector is an array detector that hasa width of 26.6 mm and a height of 6.7 mm. Such a detector is formed ofan array of pixels arranged as rows and columns. In this example atypical pixel may be 26 microns in width and height, though detectorswith smaller resolutions on the order of 10 microns are also typicallyavailable.

To aid in the selection of discrete measurement locations on patternedsamples an optional camera system 565 (i.e. camera plus necessaryfocusing elements) could be employed. While there are numerous ways inwhich to integrate such a system into the reflectometer arrangement, onepossible method is to use it to capture the beam passing through thesample channel 508 and reflecting from Beam Splitter 2. When utilized inthis manner, the camera system 565 could be used to collect images atany time the sample channel 508 is in use (i.e. when Shutter 1 is open).Alternatively, a flip-in mirror could be added to the camera system totemporarily redirect some portion of the sample beam (followingreflection from the sample) to the camera. Finally, there is also theoption of introducing separate illumination and/or collection optics tothe reflectometer in order to acquire images and locate specificfeatures on the sample.

The use of an imaging spectrograph, in combination with an array-baseddetector, enables entire spectra to be collected much faster and to ahigher degree of accuracy (due to lack of moving parts) than with aconventional scanning monochromator and single element detectorarrangement. In addition, it enables high quality imaging reflectometry,allowing data from small regions on the sample to be readily collectedand spatially resolved. This permits measurements to be performed onactual patterned production samples and not just on blanket “test”substrates or wafers. In fact, the combination of imaging optics and ahighly sensitive detection system enables multiple measurements to beperformed simultaneously on a series of sites within a localized region.

FIG. 9 illustrates the fashion in which such multiple measurements areachieved. These techniques take advantage of the chosen detector being atwo-dimensional array detector. Thus for example with regard to FIG. 5,the array detector 540 may be a two dimensional array detector. The lefthand side of the FIG. 9 presents a portion of a patterned sample 550wherein with four rectangular structures 900 formed. For example, suchstructures 900 may be formed on a semiconductor substrate such aspatterned polysilicon structures, metal structures or other structuresformed on semiconductor wafers. The structures 900 may be surrounded byan un-patterned region of the semiconductor substrate (it will berecognized that the structures shown are illustrative only to aid in theunderstanding of the invention and portions or structures of a samplemay be subject to simultaneous multiple measurements as describedherein). Superimposed on the middle two features of the sample isvertical rectangle 920 defining the spatial region that is imaged ontothe spectrometer's entrance slit. While a significantly larger regionmay actually be illuminated on the sample, only the light reflected fromthe specified area will be collected by the spectrometer and hencerecorded by the detector. The width and height of this region are afunction of the slit width 930 and slit height 940 of the spectrometerentrance slit in combination with the supporting collection andillumination optics chosen. Exemplary slits may have widths on the orderof 20-30 microns and heights on the order of 1 centimeter. As shown inthe example of FIG. 9, the sample and slit may be considered as formedof rows and columns (a row being from left to right on the page such asrows 950, and columns being from top to bottom on the page). Theinformation from the sample that passes through the entrance slit of thespectrometer is then diffracted by the diffraction grating and thenpresented to the array detector. The row information of the samplephysically maps to rows on the detector, however, the column informationdoes not as the diffraction grating disperses the column informationsuch that given wavelength components originating from all columns willmap to a single column on the detector. As a result, data correspondingto different vertical positions (i.e. rows) on the sample are imagedonto different vertical positions (rows) of the detector.

As the detector is comprised of a series of pixel rows (typically 256,512 or 1024) each individual row of pixels will record datacorresponding to different discrete locations on the patterned sample.This point is illustrated on the right hand side of FIG. 9 whichpresents reflectance spectra plots 960 collected from five separate rowsites 950 on the sample 550. Thus, for any given row site 950 of sampleinformation a spectra plot for a range of wavelengths may be obtained.Moreover, the array detector may simultaneously collect information frommultiple separate row sites 950. Thus, data for multiple wavelengths andfor multiple row sites may be simultaneously collected. The resolutionof the individual rows of sample sites that may be detected is dependentupon the pixel height utilized in the array detector. Through selectionand/or adjustment of collection and illumination optics, entrance slitwidth and detector binning configurations, a range of measurement sitesof various sizes may be achieved. In this fashion a two dimensionalregion of a sample may be illuminated by the optical path and the datafrom the two dimensional region may be recorded on the two dimensionalarray detector. As shown in the spectra plots 960 of FIG. 9, suchtechniques may be utilized to characterize the sample structures 900and/or distinguish the sample structures 900 from unpatterned regions ofthe sample. Further, though the slit width is shown as mapping a givenrow only upon the sample structure 900, the sample may be moved (left orright in the figure) such that the a given row of the slit widthoverlaps both patterned and unpatterned regions thus provided dataindicative of a combination of both regions.

The ability to simultaneously collect data from a number of discretelocations within a given localized region provides advantages withregards to measurement throughput since a significant portion ofmeasurement time per site in a conventional instrument results fromsample placement (i.e. precise adjustment and positioning of samplesites into measurement location). Additionally, this unique capabilitymay also prove useful in applications where comparative measurementsbetween closely separated sites are of interest. Typical examplesinclude, but are not limited to, dishing and erosion studies relating tochemical mechanical polishing applications. Thus, rather than having totake separate multiple measurements performed in conjunction withmovement of the sample, a single measurement may return data thatrelates to multiple positions in a two dimensional area of the sample.It will be recognized that in such techniques the quality of the opticalelements (such as mirrors, beam splitters, etc.) should be such that alarger distortion free area is provided as compared to applications inwhich two dimensional measurements are not being utilized. Thus, theoptical VUV reflectometer system provided herein may also becharacterized in one embodiment as a two dimensional reflectometersystem. It will be recognized that many uses of such two dimensionaldata collection will advantageously be utilized with a camera element asdescribed above such that pattern recognition of the two dimensionalsample area that is being analyzed may occur.

The systems and techniques described herein are particularlyadvantageous for use in applications where a high speed measurement isdesirable. In addition to the capability of obtaining data from a numberof discrete locations within a given localized region, thesemeasurements may be obtained without the need for slow step and scantechniques that utilize movable diffraction gratings.

As a result of the absorption issues discussed earlier, smallenvironmental perturbations can significantly influence measured data atVUV wavelengths. Along these lines, it is desirable to provide anapparatus which can perform measurements in a short time period in orderto minimize deleterious effects resulting from environmental changesoccurring during the measurement process. Furthermore, it is desirableto provide a means by which the measured data can be referenced to aknown standard for the purposes of data normalization. Additionally, themeans of referencing should be such as to further minimize and/oreliminate altogether errors introduced by data altering environmentalchanges which may occur between the conclusion of a calibrationmeasurement and the commencement of subsequent sample measurements.

Referencing is necessary to ensure that changes in the system (i.e.output of source, environmental conditions, etc.) are properly accountedfor and do not result in inaccurate data. While essential to ensurestability of reflectometry results in any wavelength regime, referencingis of more importance when operating in the VUV due to the loweravailable photon flux and the heightened sensitivity of the recordeddata to the composition of the gaseous medium contained within theoptical path.

Referring again to FIG. 5, in the VUV apparatus describe herein, datareferencing is accomplished through use of a reference beam channel 506.As described more herein, it is desirable that the reference beamchannel be balanced (or of the same beam length) as the source beamchannel 508. This reference beam channel 506, illustrated in FIG. 5, iscreated at Beam Splitter 1 as the source beam is split into sample andreference components. This beam is transmitted through the beam splitterand reflected off Mirrors 6, 7 and 8 before reflecting off Mirror 9. Thebeam then reflects off Beam Splitter 2 to thereafter follow theidentical path to the detector as described for the sample beam channel508 earlier. Controllable apertures may be utilized to selectivelyenable or disable the reference beam channel and the sample beamchannel. For example, the apertures may be formed from controllableoptical shutters. During the reference measurement Shutter 1 is closed,while Shutters 2 and 3 remain open.

To one skilled in the art it will be apparent that if the optical pathstraveled by the sample and reference beams from Beam Splitter 1 to BeamSplitter 2 are precisely adjusted such that they are near-identical inlength they form the two arms of a near-balanced Mach-Zehnderinterferometer. It will also be readily apparent that there exist manyother equivalent arrangements incorporating other interferometer designsfor achieving this objective. An example of one such alternateembodiment is illustrated in FIG. 10, wherein a Michelson interferometeris incorporated into the design. Though not shown other elements of thesystems of FIG. 2 or 5, such as coupling mechanisms, a camera, a purgeor vacuum system, a processor, etc. may be incorporated with the use ofthe system of FIG. 10. In the arrangement of FIG. 10, light from thesource 1010 is collimated by Mirror 1 and directed towards the BeamSplitter 1020 where the sample beam 1030 and reference beam 1040 aresplit. The sample beam 1030 travels through Shutter 1 and is focusedonto the sample 1050 by an off-axis parabolic reflector (Mirror 2).Light from the sample is captured by the same optic and travels backalong its original path. The beam then travels through the Beam Splitter1020 and is focused onto the entrance slit 1060 of the spectrometer 1070by another off-axis parabolic reflector (Mirror 3) and finally directedtoward an array detector 1080. During the sample measurement Shutters 1and 2 are open, while Shutter 3 remains closed.

During the reference measurement, the reference beam 1040 passes throughthe Beam Splitter 1020 and Shutter 3 before it is reflected back alongits path by Mirror 4. It then reflects off the Beam Splitter 1020 and isfocused onto the entrance slit 1060 of the spectrometer 1070 in asimilar fashion to the sample beam. During the reference measurementShutters 2 and 3 are open, while Shutter 1 remains closed.

The benefit of these reference configurations can be described asfollows. As the attenuation of VUV photons due to absorbing atmosphericspecies is a function of optical path length (the longer the path, themore absorbing molecules encountered), and as the dependence isnon-linear in nature, it follows that the sample and reference armsshould be substantially of the same length if similar attenuationeffects are to be encountered by each beam. If this is not the case, andthe arms are of different lengths, then data taken at any time followinga calibration measurement will only be accurate if the concentration ofabsorbing species in the environment is precisely identical to thatpresent when the calibration measurement was performed. As thiscondition is virtually impossible to ensure it remains highly improbablethat accurate results can be obtained unless the sample and referencepath lengths are equal.

As described in more detail below, providing a reference beam allows formeasurements to be obtained that indicate conditions of the opticalreflectometer system. For example, the presence of absorbing gaseswithin the optical reflectometer system can greatly affect the dataobtained from a particular sample. The reference beam channel provides amechanism that is indicative of environmental or other systemconditions. The data obtained from the reference channel may then beutilized to adjust or correct the data obtained from a sample. Thus, theuse of a reference beam to provide a mechanism to indicate theenvironmental conditions of the optical path allows for increasedaccuracy in calculations made from the data obtained from the opticalmetrology system. In addition, the use of a reference beam may allowsuitable sample data to be obtained over a wide range of environmentalconditions, thus lessening the environmental criteria, particularly forlower wavelength measurements.

In addition to ensuring that highly accurate reflectance data isobtained, the reference channel arrangement also provides a number ofother direct benefits. Firstly, the referencing scheme extends the rangeof acceptable environmental operating conditions over which reliable andaccurate data can be obtained. Quite simply, as long as theconcentration of absorbing species is sufficiently low enough to permita measurable fraction of VUV photons to leave the source, reflect fromthe sample and reach the detector, accurate measurements can beperformed. This reduces the requirements on the controlled environmentsand makes data collection over a wider range of conditions possible. Inessence the reference method enables accurate measurements to beperformed over a wide range of suitable, as opposed to reproducible,environments. As well, the interferometer approach herein described, notonly balances the path lengths of the channels, but also acts to balancethe spectral intensity profiles seen by the detector. This is importantas it allows for longer integration times and helps to mitigate anynon-linearity effects which may be inherent to the detector.

In yet another embodiment of the invention virtually all of the opticalelements, with the exception of the sample itself, are housed within theinstrument chamber. This configuration, illustrated in FIG. 11,significantly reduces the spatial requirements of the sample chamber,rendering it well suited to integrated process control applications. Asshown in FIG. 11, an optical reflectometer metrology tool 1100 isprovided. A source 1110, spectrometer 1170, and array detector 1180 areprovided within an instrument chamber 1102. Also provided within theinstrument chamber are all of the optical elements of both the samplebeam path and the reference path. Thus, mirrors 1-6, and shutters 1-3are all located within the instrument chamber 1102. Mirror 2 focuses thebeam down (into the plane of the figure) through a coupling mechanism1106 into the sample chamber 1104. From the sample 1150, the sample beamthen travels up (out of the plane of the figure) through the couplingmechanism to mirror 3. As shown in FIG. 11, the reference beam pathpasses through two coupling mechanisms 1105A (such as windows or gatevalves) that couple the reference beam from the instrument chamber 1102into the sample chamber 1104 and back into the instrument chamber 1102.In this manner the reference beam is subjected to the environment of thesample chamber just as the sample beam is. Ideally, the distance thatthe reference beam travels in the sample chamber 1104 will match thedistance that the sample beam travels in the sample chamber. Further, itwill be noted that the reference beam passes through a coupler mechanismtwice just as the sample beam does. Thus, the optical path of thereference beam is designed to closely simulate the conditions of thesample beam. In this manner, the optical paths of the reference beam andthe sample beam are similar both overall and with regard to theindividual paths in the instrument chamber and the sample chamber. Itwill be recognized that the path and arrangement of coupling mechanismshown in FIG. 11 is exemplary and other paths and arrangements may beutilized while still achieving the benefits described herein.

FIG. 11 a illustrates the FIG. 11 arrangement of the instrument chamber1102 containing mirrors 2 and 3, the coupling mechanism 1106, and thesample chamber 1104 that contains the sample 1150. As shown in FIG. 11a, the sample beam 1107 and the reference beam 1109 travel through thesample chamber 1104. It will be recognized that although perhaps lessdesirable, a system may be configured such that the reference beam doesnot pass through the sample chamber. Such a configuration may beutilized, for example, when the path length traveled by the sample beamin the sample chamber is sufficiently short and where the concentrationof absorbing species in the sample chamber are sufficientlywell-controlled, with respect to an initial calibration time and a latersample measurement time, so that errors introduced by such aconfiguration are within an acceptable error tolerance. In such a case,the reference beam may be configured so that both the reference beam andthe sample beam travel the same optical distance in the instrumentchamber. As the reference beam only travels in the instrument chamberthe total beam path will thus be different. In this manner theenvironment that the two beams are subjected to are still generallymatched (except with regard to the path length in the sample chamber).This condition may be realized in cases where the sample chamber ispurged with a high quality non-absorbing gas or where it is evacuatedusing high vacuum equipment.

The systems of FIG. 11 and FIG. 11 a may be utilized as stand-alonetools or may be integrated with another process tool. In one embodiment,the system of FIG. 11 a may be merely attached to a process tool withsome mechanism that allows for transport of the sample between theprocess tool and the metrology tool sample chamber. FIG. 11 b, however,shows an alternative manner for integrating the optical reflectometermetrology tool with a process tool. As shown in FIG. 11 b, theinstrument chamber 1102 is coupled to a coupling mechanism 1106. Thecoupling mechanism 1106 may be for example a window. In this case thecoupling mechanism 1106 may be a gate valve that is formed on a processtool 1105 or some other mechanism that allows the environment of theprocess tool 1105 to be shared with the sample chamber 1104. As shown inFIG. 11 b, the sample 1150 need not leave the environment of the processtool, rather the sample 1150 may be contained within a region 1175 ofthe process tool. Region 1175 may be a processing chamber, a transportregion or other region within the process tool. In the example shown,when the coupling mechanism 1106 (such as a gate valve) is opened, theenvironment between the region 1175 and the sample chamber 1104 isshared (note although called a sample chamber, sample chamber 1104 neverreceives the sample but rather has an environment that is shared withthe region that contains the sample). Alternatively opening the couplingmechanism may be considered to effectively expand the sample chamber1104 to include the region 1175. In this manner environmental conditionssuch as the concentrations of absorption species may be similar betweenthe region 1175 and the sample chamber 1104. The beam paths for thereference beam 1109 and the sample beam 1107 may be again designed to beof similar length within the common environment of the region 1175 andthe sample chamber 1104. The mechanism of FIG. 11 b is also advantageousin that integration may be accomplished with the sample tool byproviding a single simple coupling mechanism, such as a gate valve. Asnoted above, if the environment within the region 1175 can be closelycontrolled, it may be possible to achieve measurements within anacceptable error tolerance without sharing the environment between thesample chamber and the region 1175. In such a case, the couplingmechanism 1106 may be a window and the metrology tool would not need asample chamber 1104.

The process tool 1105 of FIG. 11 b may be any type of sample processingmechanism, such as for example, a deposition process tool, an etchprocess tool, a photolithography process tool, a planarization processtool, etc. In this arrangement, the sample will be contained within theprocess tool 1105. The process tool may contain a sample that is locatedin an optical path that may be accessed by the beam through the couplingmechanism 1106. The sample may be a located in a process tool samplechamber that is dedicated for use for metrology measurements or may belocated within some other region of the process tool. In theconfiguration of FIG. 11 b, the optical reflectometer metrology tool1100 thus may be a separate add on unit that is comprised of aninstrument chamber 1102 (and associated elements) that may be connectedto a process tool 1105 that has a coupling mechanism 1106. Theconfiguration of FIG. 11 b is advantageous in that the optical metrologytool is easily adaptable for use with a wide variety of process toolsbecause the process tool manufacture need only provide a couplingmechanism on the process tool without having to incorporate significantmetrology elements within the tool itself.

With the arrangements of FIGS. 11, 11 a and 11 b, the optical pathlength within the sample chamber can be quite short, relative to thatenclosed within the instrument chamber. In a preferred embodiment theoptical path in the sample chamber could be short within the range ofmicrons. Alternatively, to ease design of the process tool the pathcould be much longer in the range of hundreds of centimeters. The longerthe optical path, however, the more desirable it is to minimize thepresence of absorbing features and thus increase the environmentaldemands placed upon the sample chamber. If a short optical path isutilized, the requirements on the quality of the sample chamberenvironment are reduced, thereby reducing settling times and increasingsample throughput. A further benefit arises as optical surfaces housedwithin the continuously maintained instrument chamber are lesssusceptible to contamination than if they were resident in the cyclicenvironment of the sample chamber. While not explicitly denoted in FIG.11, it is implied that the optical path lengths of the reference andsample beams are near identical either through judicious design of thesample chamber itself or by some other means of adjustment orpositioning of the sample, or one or more of the coupling mechanismsbetween the sample and instrument chambers. FIGS. 11, 11 a, and 11 billustrate the use of a sample chamber of reduced size. It will again berecognized that other features and elements of the systems of FIGS. 2,5, 11, 11 a, and/or 11 b may be interchanged with each other even thoughall of such features or elements are not illustrated within the figures.Thus, for example an optical reflectometer metrology tool of FIG. 11 mayutilize camera, a purge or vacuum system, a processor, a Michelsoninterferometer design, etc. and it will be recognized that the systemshown in any particular figure is not limited to use with only thoseelements illustrated or the arrangement of the elements as shown.

The beam splitters employed in the device can be of various designs. Forexample, the beam splitters may be partially transmissive beam splittersthat obscure the entire beam diameter or fully reflecting mirrorsobscuring some portion of the entire beam diameter. If operation atwavelengths above 115 nm is desired and VUV photon flux is sufficient,then conventional thin film interference beam splitters employing MgF₂substrates can be utilized. A particularly well-suited beam splitter forthis application is produced by Acton Research Corporation (ModelVUVBS-45-MF-2D). Typical reflectance and transmittance properties forthis beam splitter are presented in FIG. 13 as plots 1310 and 1320.Plots 1310 and 1320 respectively illustrate the % reflectance at 45° andthe % transmission at 45° as a function of wavelength. If operation atwavelengths below 115 nm is desired, or if photon levels aresufficiently low enough, a spatial beam splitter (totally reflectingmirror bisecting the optical path) or flip in mirror approach (replaceBeam Splitters 1 and 2 with flip in mirrors and eliminate Shutters 1 and2) can be used.

Mirrors 6, 7 and 8 provide a means of adjusting the path length in thereference arm such that it closely parallels the path length in thesample arm such as for example as shown in FIG. 5. Those skilled in theart will recognize that there exist many alternate means ofaccomplishing this objective. For example, a configuration as shown inFIG. 11 may be utilized in which mirrors 7 and 8 are absent. Thebenefits associated with this effort are apparent upon examination ofFIG. 14 where the difference in absorbance between the sample andreference arms is plotted for different path length differences andconcentrations (in PPM) of O₂ and H₂O contaminants. FIG. 14 plots thedifference in concentrations of the contaminants from an initial timewhen the system is calibrated and at a later time when an unknown sampleis measured versus the differences in path lengths of the sample andreference channels of the instrument. The plots 1410, 1420 and 1430correspond to lines of absolute errors of 0.01%, 0.10%, and 1% that willbe introduced in the measured reflectance data for a specific set ofconcentration and path length differences. Thus, for example, point 1445illustrates that an approximate absolute error of 0.01% may result ifthe concentration difference between the time the calibration sample ismeasured and the time the sample measurement is measured is between1.0E+01 and 1.0E+02 and the path length difference is approximately 0.01centimeters. It will be recognized that FIG. 14 is an exemplary graph todemonstrate the principles described herein. For example the graph inFIG. 14 assumes that the O₂ and H₂O change together (i.e. aconcentration difference of 10 PPM corresponds to a 10 PPM change in O₂and a 10 PPM change in H₂O). Further, it will be recognized that othercontaminants that are absorbing species may be present. In addition,though FIG. 14 presents data at a wavelength of 145 nm, otherwavelengths will similarly reflect the concepts described.

Thus if a particular application requires that errors be held below 0.1%and if the concentration of absorbing species in the sample chamber canbe expected to change on the order of 100 PPM between the initialcalibration and the final sample measurement time, then as reflected inFIG. 14 a maximum path length difference may be calculated. In theexample presented, such path length difference may be less than about0.025 cm. If the expected concentration differences are expected to belarger, the acceptable path length differences will be reduced.Likewise, if error must be held lower than the acceptable path lengthdifferences (for a given concentration difference) must be lower. It isnoted that these effects are highly dependent upon the presence ofabsorbing species in the ambient and that the absorbance differences fora given path length difference increase non-linearly as the environmentdegrades.

While different applications can sustain different degrees ofinaccuracy, it is likely that in many applications one would generallyprefer to keep such errors less than 0.1% and in some cases below 0.01%or less. The range of concentration differences that could beencountered would depend to a large extent on how the instrument wasdesigned and employed. For example, stand alone systems may be designedfor use with adequate purge and/or vacuum control such that it is likelythat concentration differences could be maintained at very low levels(on the single digit PPM level), whereas in integrated applicationswhere the metrology instrument is attached to other process tools suchas described with reference to FIG. 11 b (and hence some fraction of thesample chamber resides within that other process tool), it may not bepossible to control the differences.

During both the sample and reference measurements Shutter 2 of theembodiments described above with respect to FIG. 5, 10 or 11 acts toaccurately control the duration of the measurement, which directlyimpacts the accuracy of the measured data. As such, Shutter 2 ispreferably a high speed electronic shutter that can be preciselycontrolled on the millisecond timescale. An example of such a shutter isModel 76994 manufactured by Thermo Oriel of the United States. Shutter 2also acts to prevent light from the source from reaching opticalsurfaces in the instrument during times when measurements are notactively underway in order to prevent changes in those surfaces whichmay result from prolonged exposure to the light from the source.

It is important to note that using the designs presented herein, signalsfrom both the sample and reference channels are dispersed using the sameregion of the diffraction grating within the spectrometer and arerecorded using a common detector. This helps to avoid inaccuraciesresulting from differences in the local performance of a grating anddifferences in responses between multiple detectors.

Additionally, it is desirable that a means exist for adjusting or tuningthe angles upon which the sample and reference beams enter thespectrometer, such that the two beams are coincident. Differencesbetween the entrance angles of the two beams may result in complicationsincluding, but not limited to, artifacts and unwanted features in theratio of the two signals owing to different effective spectralresolutions (since the two beams “see” different effective slit widths).An effective means of adjusting the entrance angles can be providedthrough use of a standard kinematic mounting apparatus with which tohold Beam Splitter 2. One skilled in the art will recognize that manyother means of adjusting the entrance angles could also be employed. Asmentioned above, it may be desirable to closely match the optical pathdistance of the sample and reference beams such that near equal opticalpath lengths are obtained. It may be desirable to also match the numberand types of optical elements such as mirrors, beam splitters, etc. sothat reference and sample paths having substantially similarcharacteristics are provided. However because absorption resulting fromenvironmental conditions of the chamber may be a dominating factor, theoptical path distance may be the most critical factor in matching thebeam paths.

In view of the challenges presented by environmental absorption, it isdesirable to reduce the overall path length of the device to as short asis practically possible. Limitations on the extent to which this designparameter can be optimized will depend on a number of systemcharacteristics including, but not limited to, the brightness of thesource and spectral resolution required. Moreover, it is also beneficialto reduce the volume of the instrument so as to minimize the settlingtime and quantity of purge gas required to purge the instrument and/orsample chambers. Both of these characteristics can be expected to beinfluenced to some extent through introduction of forced circulation andintelligent mechanical design to ensure sufficient mixing of gasesoccurs.

The controlled environment of the instrument brings with it a number ofrelated benefits. Firstly, the use of vacuum or high purity purgeconditions necessarily implies an absence of potential contaminateswhich could lead to oxide growth, hydrocarbon build up, moistureadsorption and the like. This consideration becomes increasinglyimportant as leading edge wafer processing techniques incorporatethinner layers and smaller features, which are now comparable to and/orsmaller than the dimensions associated with the thickness of filmsinadvertently created through contamination processes. In applicationswhere ultra-thin layers are involved it is likely that improvements inmeasurement accuracy may be realized through pre-measurement treatmentof samples in an optional desorber unit (see FIG. 2) in order to removecontamination layers which may be present. As known in the art, such adesorber may remove moisture and other contaminates such as hydrocarbonsby using thermal heating. This capability would also play a key role inensuring the accuracy of calibration and test materials. A furtherbenefit of the controlled environment is that it would provide excellentmeasurement stability, as the temperature and particulate levels withinthe instrument could be well controlled.

It may be noted that the referencing techniques described herein provideadvantages beyond traditional calibration techniques and that thereferencing techniques may be used in combination with calibrationtechniques and/or in place of calibration techniques. In traditionalcalibration techniques a reference having a known characteristic (suchas a known reflectance) is provided for measurement. The measurementfrom the known sample is then utilized to assist in analyzing the datathat is obtained from a measurement taken from the unknown sample. Suchcalibration techniques, however, are time consuming if a calibration isperformed before every measurement is made on an unknown sample(particularly if multiple measurements are performed upon each sample).In addition, calibration itself introduces errors in that the quality ofthe calibration sample may degrade overtime (for example as thecalibration sample becomes contaminated over time). In addition, themovement of a calibration sample into and out of a sample chamber mayintroduce further environmental changes that affect the accuracy of thedata analysis. The reference techniques described herein may beaccomplished without the mechanical introduction of error.

The referencing technique provided herein may be performed, however,quickly and with minimal system impact. Thus for example, a referencemeasurement may be easily obtained prior to every measurement collectedfrom a sample. Thus real time data referencing closely in time to thesample data collection may be obtained to indicate the conditions of themetrology system. Further, this referencing data may used to adjust thesample data because system absorption effects have been detected. Inaddition this reference data may truly characterize the system withoutbeing dependant upon a standard sample. This referencing data may alsobe used to adjust the sample data because other system changes haveoccurred (i.e. like changes in source output). The referencingtechniques also may be combined with traditional calibration techniquesto more fully characterize collected data. Thus, a system calibrationmay be performed upon some periodic basis (once a day, once a week,etc.) and the reference techniques may be performed significantly morefrequently, for example once per sample or prior to every measurementtaken from a sample.

An example of the typical steps involved in a calibration, referencingand measurement sequence 1200 are provided in a high level in FIG. 12.As shown by step 1202 a calibration sample may be loaded into a samplechamber and a suitable system state (such as absorbing species) may beestablished. Then, measurements may be performed on the knowncalibration sample in order to calibrate the optical reflectometersystem as shown by step 1204. It will be noted that the system may beactually calibrated at this time or the calibration data may be merelycollected to be utilized to adjust any final data results presented frommeasurements made on an unknown sample (such as adjustments implementedthrough subsequent software algorithms). Measurements of the referencechannel may then be obtained as shown by step 1206 in order tocharacterize and record the state of the reflectometer system at thetime that the calibration measurement was performed. It will be notedthat as shown, the measurements of the referencing channel are shown tobe performed after the measurement on the calibration sample, however,the referencing measurements may be performed prior to the calibrationmeasurement. It is desirable, however, for such measurements to be maderelatively close in time so that the system characteristics at the timeof calibration may be determined.

Next an unknown sample that is desired to be analyzed may be loaded intoa sample chamber and a suitable system state (such as absorbing species)may be established as shown by step 1208. An optical reflectometermeasurement may then be obtained from the unknown sample as shown instep 1212. Measurements of the reference channel may then be obtained asshown by step 1214 in order to characterize and record the state of thereflectometer system at the time that the measurement on the unknownsample was performed. Once again it will be noted that as shown, themeasurements of the referencing channel are shown to be performed afterthe measurement on the unknown sample, however, the referencingmeasurements may be performed prior to the unknown sample measurement.Finally, as shown in step 1216, the results of the sample measurementmay be adjusted using recorded information from the referencemeasurements performed at the time of the system calibrationmeasurements and at the time of the sample measurement. Theseadjustments are made in order to remove errors resulting from changes inthe state of the system. Thus in this manner changes in theconcentration of absorbing species at the time of calibration and thetime of measurement of the unknown sample may be accounted for. Thus,the reference beam may be utilized to assist in characterizing theambient environment concentrations or differences in the concentrations,particularly where other variables such as path length differences areknown or may be accurately estimated. As will be described in moredetail with reference below to FIG. 14, the presence of non-zero pathlength differences between the reference beam path and the sample beampath will limit the accuracy of corrections that may be made due toabsorbing species concentration variations. Additionally, changes thatmay be accounted further include changes in elements of the system thatare common between the reference beam path and the sample beam pathwhich may exist between the time of calibration measurements and thetime of the unknown sample measurements. For example changes ofcharacteristic of the source, shared optics, the spectrometer, thedetector, etc. may be addressed. Such changes may be the result ofage/lifetime variations, temperature variations, mechanical variations,etc.

A more detailed example of the typical steps involved in a calibration,referencing and measurement sequence 1200 are provided in the flowchartof FIG. 12 a (split as FIGS. 12A-A and 12A-B). As indicated by step1205, a calibration sample with a known reflectance may be loaded into alocation for measurement (such as within a sample chamber) and thenpurging and/or vacuum pumping may occur to establish suitably lowenvironmental concentrations of absorbing species. An opticalreflectometer measurement may then be obtained from the calibrationsample to record an intensity of the calibration sample as indicated bystep 1210. Such data may be saved by the processor or other computingsystem. Next a source intensity profile may be calculated as indicatedby step 1215. Step 1220 includes recording the intensity of thereference channel at a time t₁. Utilizing the prior recorded andcalculated data, a reference reflectance may then be calculated as shownin step 1225.

Next the unknown sample may be loaded into the system and the suitableconcentrations of absorbing species may be again obtained as indicatedby steps 1230. Another reference measurement may then be recorded andsaved as indicated by step 1235 where the intensity of the referencechannel is recorded for time t₂. The source intensity profile is thencalculated again in step 1240 using the data from step 1235. The sourceintensity profile may be rewritten as shown in step 1245. The intensityof the unknown sample may then be recorded as shown in step 1250 and thesample reflectance can be calculated as shown in step 1255. The samplereflectance may be calculated utilizing the rewritten equations of steps1260 and 1265. It will be noted that the exponential term the equationof step 1265 is written for the case of two beams (sample and reference)in a single chamber. In the more complicated case of two chambers itwould expanded to include two exponential components, one tocharacterize the differences in the first chamber and the second tocharacterize the differences in the second chamber.

Additional measurements may then be performed on the same unknown sampleor another unknown sample. It will be recognized that for suchadditional references another loading and measurement of a calibrationsample may not occur for each of such measurements, but rather, thecalibration data may be stored for re-use and only the referencing andunknown sample steps need be performed again. In yet another embodiment,the data of the referencing steps may also be reused such thatadditional referencing is not performed for every additional unknownsample measurement. Thus, it will be recognized that the referencingtechniques described herein may be utilized in a wide variety of mannerswhile still obtaining at least some of the benefits of the referencingtechniques;

As indicated in steps 1255-1265 of FIG. 12 a-b, the dependence of thepath length and concentration differences are clearly shown. As alsoindicated in step 1265, when the path length difference(Lsample−Lreference) decreases toward zero, any error caused by theexponential dependence term is reduced since when the differenceapproaches zero the exponential term approaches unity. It is noted thatthis will occur independent of the differences in the concentrations(N2−N1). In addition to the typical steps illustrated in the figure, itis recognized that a background measurement performed in the absence oflight (i.e. a measurement with both sample and reference shuttersclosed) would be recorded and subtracted from all subsequentmeasurements. By nature of the fact that the detector used in theinstrument is both cooled and temperature controlled it is unlikely thatsuch background measurements need be performed regularly as thebackground levels associated with such a detector configuration would beexpected to be low and highly stable.

It will be recognized that advantages of the optical metrology systemsdisclosed herein may be obtained without requiring the use of thereferencing techniques described above. Thus, the systems and techniquesdisclosed herein may be implemented independent of the referencingtechniques or in combination with the referencing techniques. Further,the referencing techniques provided herein may be utilized with opticalmetrology systems different from those disclosed herein or with systemsthat operate at different wavelengths. The referencing techniques andthe optical metrology systems disclosed herein, may however, beparticularly advantageous when used in combination.

While not shown in FIGS. 12 and 12 a (split as FIGS. 12A-A and 12A-B),it may also be useful in situations, where significant levels ofstray-light are present, to perform an additional corrective step duringthe data acquisition process. Stray light refers to light generatedthrough scattering processes at optical surfaces in the beam path of thesystem. The presence of such light can ultimately result in spuriouscounts recorded by the detector (i.e. light of wavelength other thanλ_(o) that is incident upon the pixel corresponding to λ₀). While theVUV apparatus described herein has been designed such to considerablyreduce the generation of stray light within the device, it may still beadvantageous to correct for this phenomena in some circumstances.

One approach to correct for stray light within the system involvesattempting to record light below the spectral range of the instrument(i.e. below the lower wavelength cutoff of the device). Any signalrecorded below this region should not, by definition exist, and isinstead assumed to have been created through scattering processes.Equipped with an understanding of the intensity of such signal, as afunction of wavelength, it is possible to subtract the appropriate“stray light” contribution from longer wavelength regions within thespectral range of the instrument where “real” signals are simultaneouslybeing recorded.

The concepts disclosed herein provide a VUV optical reflectometermetrology tool. The design of the tool is simple and robust rendering iteasy to operate at VUV wavelengths. Further, the tool avoids many of theproblems associated with ellipsometry techniques. For example, the tooland techniques disclosed herein may be utilized without polarizationelements. In ellipsometry, the change in the polarization state of lightreflected from the surface of a sample is measured. Typical ellipsometrytechniques use at least two polarizing elements (one in the optical pathprior to the sample and one in the optical path after the sample). Suchtechniques are time consuming because of the nature of collecting datafor multiple polarization angles. In addition, polarization elements aregenerally absorbing thus making them unsuitable for low wavelengthmeasurements, particularly in the VUV regions of about 140 nm or less.Thus, the systems and techniques described herein (which may be utilizedwithout polarizing elements) are particularly advantageous for use withwavelengths that are low end VUV regions (or lower). The absorbingnature of polarizing elements also increases the time necessary tocollect sufficient light to obtain a measurement.

Thus, it may be desirable to provide a reflectometer tool utilizing thetechniques disclosed herein with a non-polarizing optical path such thata polarization independent measurement may be obtained. The polarizationindependent techniques shown herein provide a phase independentreflectivity amplitude measurement. The reflectometer tools describedherein typically include multiple wavelengths within the optical pathuntil the optical path hits the diffraction grating, at which point thewavelengths are spatially separated. Ellipsometry techniquestraditionally involve filtering the light source to a single wavelengthat some point in the optical path. It should be noted that at least someof the techniques and tools described herein may be useful forapplications known as polarized reflectometry. Such applications maytypically use a single polarizing element located either before or afterthe sample to enable collection of reflectivity amplitude data in one oftwo possible polarization states.

The tools and techniques disclosed herein also are advantageous comparedto ellipsometer techniques because of the smaller angle of incidencethat is required of the optical beam with reference to the sample. Thusfor example as shown with reference to FIG. 11 a, an angle of incidenceφ of 10° or less and even 4° or less is possible utilizing thetechniques disclosed herein as opposed to ellipsometer techniques whichoften utilize angles of incident on the order of 70°. This isadvantageous as the footprint of the metrology tool is smaller and theintegration of the metrology tool with process tools is simpler. Forexample, it is possible to integrate the metrology tools disclosedherein with a process tool through the use of one coupling mechanism asopposed to requiring multiple coupling mechanisms.

Once spectral reflectance data is recorded by the detector it is sent tothe processor unit depicted in FIG. 2, where it is subsequently reducedvia analytical algorithms. These algorithms may generally relate opticaldata, such as for example reflectance, to other properties of the samplewhich can then be measured and/or monitored. If the sample is comprisedof a thin film 1505 (or stack of thin films) on a substrate 1510, thenthe situation can be depicted as in FIG. 15 and the associated sampleproperties may include quantities such as for example, but not limitedto, film thickness, complex refractive index, composition, porosity andsurface or interface roughness.

Data reduction is generally accomplished using some form of the Fresnelequations in combination with one or more models to describe the opticalproperties of the material or materials comprising the sample. There area large number of such models in existence with differing degrees ofapplicability, depending on the nature of the materials involved.Frequently used models include, but are not limited to, the effectivemedian approximation (EMA) and variations on what is commonly referredto as the “harmonic oscillator”. Regardless of the specific models usedin the reduction of the data set, the greater goal is generally to use avalid mathematical expression to describe the measured data such thatcertain parameters relating to properties of the samples (as discussedabove) can be obtained through an iterative optimization process. Thatis, the measured data set is compared to one calculated using anexpression that depends on a set of parameters relating to the nature ofthe sample. The discrepancy between the measured and calculated datasets is minimized by iteratively adjusting the values of the parametersuntil such time as adequate agreement between the two data sets isachieved. This discrepancy is usually quantified in terms of a “goodnessof fit” parameter.

As many materials exhibit significantly more structure in the VUV regionof their optical properties, than at longer wavelengths in thedeep-ultra-violet (DUV) and visible regions, there is a considerableadvantage associated with the extended data range afforded by the VUVapparatus described herein, particularly as is relates to the datareduction process. This point is illustrated through the examplesprovided in FIG. 16 and FIG. 17. The two curves in FIG. 16 represent themeasured reflectance spectra 1610 (solid line) and calculatedreflectance spectra 1620 (dotted line) of a thin (˜50 Å) aluminum oxide(Al₂O₃) layer deposited on a silicon substrate. The calculated resultwas obtained using the data reduction methods outlined above. As isevident, excellent agreement is obtained between the measured andcalculated spectra, providing a high degree of confidence in theaccuracy of the acquired results.

The n and k values (the values of the real and imaginary partsrespectively of the complex index of refraction) obtained for the Al₂O₃layer are presented in FIG. 17. As is evident from the plot 1710 of nvalues and the plot 1720 of k values, the optical properties in the DUVand visible region show little in the way of defining structure, as themain peaks associated with the n and k spectra reside exclusively atshorter wavelengths in the VUV. As the parameters in the fittingalgorithm are inherently related (among other things) to the position,amplitude and breadth of these peaks it follows that an accuratedetermination of such parameters is greatly aided by providing thefitting routine with actual measured data spanning the wavelength rangeof interest. In other words, as the optical properties of many materialstend to exhibit the majority of their defining structure in the VUV (andnot in the DUV or visible regions) it is highly desirable to make use ofmeasured data in this spectral region when attempting to accuratelydetermine such properties. FIG. 18 illustrates how the VUV techniquesdisclosed herein may be utilized to identify and measure very thinlayers in a semiconductor process environment. The first curve 1810 inthe figure, corresponding to the y-axis on the right hand side, presentsthe reflectance signal associated with an ultra-thin (5 Å) layer ofresidual photo resist on a blank mask substrate. The second curve 1820,corresponding to the y-axis on the left hand side, presents thedifference signal associated with a 1 Å increase in the film thicknessof the said layer of residual photo resist. It is clear that the largestchanges in the difference signal appear at shorter VUV wavelengths, andthat the difference signal tends to zero as the wavelength approachesthe longer wavelengths in the DUV. FIG. 19 provides a further example ofhow the disclosed methods may be used to measure or monitor thethickness of ultra-thin layers. Three curves are present in the figureand correspond to reflectance spectra recorded from samples consistingof a thin 10 Å layer (curve 1810), a thin 14 Å layer (curve 1820) and athin 18 Å layer (curve 1830) of silicon oxy-nitride (SiON) deposited onsilicon substrates. As is evident, the differences between the spectraare again greatest at the shorter VUV wavelengths and, in this case,essentially non-existent at longer DUV wavelengths. This is anincreasingly important aspect as it relates to semiconductor processcontrol since the semiconductor industry is constantly working toincorporate thinner and thinner layers into semiconductor devices.

This point is further emphasized upon examination of FIG. 20, whichpresents the reflectance change (relative to a nominal 10 Å layer) for aSiO_(0.87)N_(0.13) layer as a function of film thickness (relative to anominal 10 Å layer with 13% nitrogen). As is evident from the graph thereflectance changes at 130 nm (plot 2010) for a given change in filmthickness are larger in turn than those expected at either 157 nm (plot2020) or at 193 nm (plot 2030). In fact, the changes in the VUV at 130nm are approximately seven times greater than those exhibited at in theDUV at 193 nm. FIG. 21 and FIG. 22 illustrate generally how the VUVtechniques described herein may be utilized to monitor the compositionof a material or film. FIG. 21 presents reflectance spectra for a seriesof six 16 Å thick SiON layers deposited on Si with concentrationsranging from 10% to 15%. As is evident, region 2110 is the region ofhighest sensitivity to changes in the composition of the SiON films andis centered at approximately 130 nm. This point is further emphasizedfollowing examination of FIG. 22 which presents the reflectance change(relative to a nominal 16 Å SiON layer with 10% nitrogen) for a SiONlayer as a function of film thickness (relative to a nominal 10 Ålayer). As is evident from the graph the reflectance changes at 130 nm(plot 2210) for a given change in film thickness are larger in turn thanthose expected at either 157 nm (plot 2220) or at 193 nm (plot 2230).

As a further example of the benefits afforded by the use of the VUVmethods presented herein, the determination of the composition of a SiONfilm exhibiting a non-uniform distribution of nitrogen (as a function offilm thickness) is considered. FIG. 22 a presents a series of fournitrogen distributions for 20 Å SiO₂ films subjected to a dose of 1e¹⁵nitrogen atoms/cm². The nitrogen atomic % is plotted as a function offilm depth (as measured from the ambient/film interface). While the samenumbers of nitrogen atoms are contained within each of the four samples,the distributions of those atoms are considerably different. In one casethe nitrogen is uniformly dispersed throughout the thickness of thelayer (plot 2240), in another it is incorporated such that it exhibits abroad Gaussian distribution centered in the middle of the film depth(plot 2250), in yet another it exhibits a bottom heavy Gaussiandistribution (centered closer to the film/substrate interface) (plot2260) and in the final case it exhibits a exponentially decayingdistribution (plot 2270).

FIG. 22 b presents the reflectance difference signals associated withthe samples exhibiting the broad centered Gaussian (plot 2251), bottomheavy Gaussian (plot 2261) and exponentially decaying (plot 2271)distributions. The reflectance difference signal is obtained bysubtracting the reflectance signal associated with the normallydistributed sample from that of the other three. As is evident, thenon-uniformly distributed samples all exhibit significant and clearlydistinguishable reflectance difference signals in the VUV region of thespectrum, while at the same time exhibiting little or no differences atlonger wavelengths. This figure acts to further illustrate how the VUVtechniques herein disclosed can be used to measure and/or monitor thecompositional profile of very thin layers.

While the exemplary layers of FIG. 17-FIG. 22 b are those of Al2O3,photoresist and SiON, it will be recognized that layers and film stacksof other materials, deposited on a variety of substrates including, butnot limited to silicon wafers and photo mask blanks, may be measured ina similar fashion.

Another advantage afforded by the VUV wavelengths may be realized whenmeasuring certain film stacks comprised of two or more layers. As thenumber of films in a stack is increased, so generally is the number ofparameters sought in the optimization routine. As the number ofparameters increases so does the likelihood that correlations betweenthe parameters may exist. In some circumstances this may contribute toinaccuracy or instability in measured results. In some situations it maybe possible to simplify the problem, and hence reduce the number ofparameters sought in the optimization routine by exploiting use of theoptical data in the VUV through incorporation of an intelligentweighting function.

This function, herein referred to as the “dynamic weighting function”involves dynamically ascribing greater or lesser emphasis on specificdatum during the optimization process depending on their expectedcontribution to the determination of the set of parameters being sought.In such an approach the expected contribution is dynamically estimatedbased on the expected configuration of the sample (i.e. thickness andcomposition of layers comprising the sample) and is updated on aniteration by iteration basis. For example, as shown in FIG. 23 whenmeasuring a two layer film stack comprised of silicon dioxide (SiO₂)layer 2310 and silicon nitride (SiN) layer 2320 deposited on a siliconsubstrate 2340, it may prove beneficial to place greater emphasis ondata points in the VUV during the search for thickness of the top SiO₂film. This follows from the fact that the SiN is, for all intents andpurposes, opaque to VUV photons at thicknesses greater than about 1000Å. Thus as shown in FIG. 23, reflectance 2350 from the SiN-Substrateinterface may be present in measurements made with DUV wavelengths butmay be absent in measurements made with VUV wavelengths. Hence, thethickness of the underlying SiN layer can essentially be disregardedduring the optimization process if DUV and longer wavelength data isneglected. This point is further illustrated upon examination of FIG. 24and FIG. 25. FIG. 24 presents reflectance data from three SiO₂/SiN/Sisamples. The SiN layer thickness varies from 1000 Å (plot 2410), to 2000Å (plot 2420) to 3000 Å (plot 2430) amongst the samples, while the SiO₂layer thickness remains fixed at 10 Å. As is evident, the reflectancespectra from the three samples appear markedly different in the DUVregion, and yet virtually identical at VUV wavelengths. This followsfrom the fact that VUV photons do not penetrate the SiN layer andinstead “see” a sample comprised of 10 Å of SiO₂ deposited on a SiNsubstrate. Applying a weighting function which strongly emphasizes theVUV and strongly de-emphasizes the DUV and longer wavelength datathereby reduces the parameter set sought by the optimization routinesince the result is then insensitive to the SiN layer thickness. Thisapproach reduces or altogether removes any correlation between thethickness parameters for the SiO₂ and SiN layers that may exist, therebyacting to increase the accuracy and repeatability of the measurementresults. Additionally, this approach will generally result inconvergence of a solution in a significantly shorter period of time thanpossible using conventional methods.

Further evidence of the benefit of such a dynamic weighting function isprovided in FIG. 25, which also presents reflectance spectra from threeSiO₂/SiN/Si samples. In this case the SiN layer thickness is fixed at1000 Å amongst the samples, while the SiO₂ layer thickness varies from 0Å (plot 2510), to 10 Å (plot 2520) to 20 Å (plot 2530). As is seen, thespectra exhibit clear differences in the VUV region, while appearingvirtually identical in the DUV. Thus, because of the sensitivity of thetools and techniques described herein to absorption effects, theabsorption of shorter wavelengths in the thin films being measured maybe advantageously utilized. Moreover, in situations in which a roughestimate of the anticipated sample characteristics is known (for examplea rough estimate of the underlying SiN film thickness), greaterimportance (or dynamic weighting) may be placed upon the reflectivitydata in certain wavelength regions.

While the exemplary samples of FIG. 23, FIG. 24 and FIG. 25 arecomprised of SiO₂/SiN/Si is it clear that the dynamic weighting functionapproach can be used to measure and or monitor samples possessing morethan two layers and which, are comprised of different materials.

The dynamic weighting function may also be utilized in conjunction withan iterative data fitting process. For example, for data collected fromthe SiO2/SiN/Si layers described above with reference to FIGS. 23-25, aniterative process can be utilized to attempt to determine the thicknessof each of the layers. During each iteration of the fitting routine thedifferences between the calculated and measured data sets may bemathematically compared at each wavelength and used to determine whetherchanges made in the values of the parameters of the fitting routine (inthis case the film thicknesses) were an improvement over the parametervalues obtained in the previous iteration. It is advantageous to includea weighting factor which takes into account the approximate nature ofthe sample. For example, the data in FIG. 25 clearly reveals thatwavelengths above ˜180 nm contain no information about the thickness ofthe top SiO2 layer. Traditional data fitting methods would ignore thisfact and attempt to compare the measured and calculated data at allmeasured wavelengths when seeking this thickness. As a result, most ofthe wavelengths being compared (those greater than 180 nm) could onlyincrease the uncertainty into the result since they represent asignificant portion of the weighted comparison function. Using thedynamic weighting function approach the problem could be broken downsuch that only measured data that could reasonably be expected tocontain useful information would be included into the weightedcomparison function. The method is dynamic since the decision makingprocess (which measured data should be considered) could be repeatedafter each iteration.

When patterned samples are involved additional theoretical constructsare typically invoked to properly describe the light scattering, whichoccurs as a result of the interaction between the measurement photonsand periodic patterned features. Such light scattering is shown withreference to FIG. 26. FIG. 26 illustrates a patterned substrate 2610 andthe reflected beam 2620 and diffracted beams 2630 that result from theincident beam 2640. This form of non-imaging optical dimensionalmetrology is known as scatterometry and commonly involves employment ofsome form of “rigorous coupled wave analysis” (RCWA) during the datareduction process. This technique exploits the sensitivity of lightscattering from a patterned sample and relates the dimensions of thefeatures comprising the sample to the optical signal recorded from suchthrough use of an appropriate mathematical expression. In other words,scatterometry enables the dimensions of patterned features to bedetermined by accounting for light scattered or diffracted from a samplecontaining patterned features.

Examples of exemplary quantities that may be measured and/or monitoredon a patterned substrate 2700 using this approach are depictedgraphically in FIG. 27 and include, but are not limited to, criticaldimensions (line widths) 2710, sidewall angles 2720, trench depths (orline height) 2730, trench widths 2740 and film thickness 2750. It isunderstood that these quantities represent a select number of the manysuch quantities that may be measured and/or monitored in thin filmstacks and/or structures. Patterned thin film samples of this nature arefound in many areas including semiconductor devices and storage media.

As a review of light scattering physics reveals, short wavelengthphotons like those in the VUV are inherently better suited to measuringor monitoring smaller critical dimensions of patterned features thanlonger wavelength photons, owing to the increased sensitivity affordedby the former. In fact, it can be seen that for many critical dimensionmetrology applications involving leading edge semiconductor devices,measurement is only possible using short wavelength VUV photons. Thispoint will is further illustrated through the examples provided below.

FIG. 28 illustrates an exemplary VUV measurement relating to line widthdetermination. The first curve 2810 in the figure, corresponding to they-axis on the right hand side, presents the reflected signal obtainedfrom a 65 nm line array with a pitch of 130 nm. That is, a line arrayconstructed such as to exhibit 65 nm wide lines separated by 65 nm widespaces. The second curve 2820 in FIG. 28, corresponding to the y-axis onthe left hand side, presents the difference in the reflected signalbetween 66 nm and 65 nm line arrays. That is, this curve represents thedifference signal associated with a 1 nm increase in line width for aline array exhibiting 65 nm wide lines and spaces. As is evident fromthe figure, distinctive and significant changes in the difference signalare only expected at and below wavelengths corresponding to the pitch ofline array (65 nm line width+65 nm space width=130 nm pitch). Hence, inorder to measure or monitor the line width in such a structure using theapproach described herein, it is necessary that the range of measuredwavelengths include those at and below the pitch wavelength.

FIG. 29 illustrates an exemplary VUV measurement relating to pitchdetermination. The three curves in the figure represent reflectancesignal expected from line arrays comprised of 63 nm (curve 2910), 65 nm(curve 2920) and 67 nm (curve 2930) lines and spaces. That is, the datarepresent signal from line arrays with equal line width and space width,but with pitches of 126 nm, 130 nm and 134 nm. As is evident from thefigure, changes in the three spectra are predominantly evident in thespectral region immediately at and below the wavelength corresponding tothe line array pitch (again near 130 nm in this example).

FIG. 30 illustrates how the VUV techniques and apparatus describedherein can be used to measure or monitor changes in the height of linescomprising a line array. Two curves are presented in the figure. Thefirst curve 3010, corresponding to the y-axis on the left hand side,presents the expected reflectance signal from a line array with 65 nmlines and spaces, wherein the line height is 1000 Å. The second curve3020, corresponding to the y-axis on the right hand side, presents thedifference signal associated with a 10 Å increase in line height for thesame such line array. As is evident the changes in line height bringabout a spectral signature markedly distinct from the changes introducedthrough in line width and pitch presented earlier (for reference referto FIG. 29 and FIG. 30). That is, the spectral region exhibiting thesmallest difference signal resulting from changes in line height is infact the same spectral region exhibiting the largest difference signalresulting from changes in line width and pitch.

Application of the VUV techniques and apparatus described herein in thefield of semiconductor process control metrology are both numerous andwide-ranging. In general, it has been demonstrated that the VUVreflectometer techniques provided herein may provide data shownreflectance magnitudes at given wavelengths. Further, the sensitivity ofthese measurements may be meaningfully related to semiconductormanufacturing process data to provide data indicative of various processvariables. In this manner the systems and techniques provided herein maybe utilized in process control and process characterizationapplications. Specific examples of a select number of such cases havebeen presented, however those skilled in the art will recognize thatthese methods can be further applied in many other situations.

The techniques described herein may be incorporated into off-line standalone metrology equipment utilized for metrology applications. However,because they may be implemented in a relatively less complex hardwaresolution that may yield a measurement result relatively quickly andrepeatedly, the techniques described herein may be particularly suitedfor incorporation into any of a wide variety of semiconductor processtools. Thus, for example, the VUV techniques described herein may beincorporated directly into tools used for deposition, etch,photolithography, etc. so that in-line measurements, monitoring andcontrol may be advantageously obtained.

The equipment, components, materials, and techniques described above maybe utilized in a system that utilizes a broad band range of wavelengths.For example, a reflectometer that includes VUV wavelengths may beconfigured to operate in at least one other spectral region. Thus, allor parts of the systems and techniques described above with reference toFIGS. 1-30 may be utilized in conjunction with broad band systems andtechniques. FIGS. 31-39 and the associated text below describe variousbroad band systems and techniques that may be utilized in conjunctionwith the equipment, components, materials, and techniques describedabove.

A simplified representation of one embodiment of a broad band system3100 is presented in FIG. 31. In operation, light from one of threesources 3102, 3104, and 3106 is selected, directed and focused by theoptical module 3108 onto the surface of the sample 3110. Upon reflectionfrom the sample the light is again collected and directed to one ofthree detectors 3112, 3114, and 3116, as selected by the optical module3108. In certain circumstances the optical module may also provide acontrolled environment between the sources, sample chamber 3120 anddetectors. Additionally, in some cases the optical module may act toimprove system performance by providing a means by which collected datais referenced. The optical module is controlled by a processor 3122,which may also be used to analyze data recorded by the detectors.

FIG. 32 presents an embodiment 3200 of the invention configured tocollect referenced broad band reflectance data in both the VUV andDUV-NIR. In operation light from these two spectral regions is obtainedin a serial manner. That is, reflectance data from the VUV is firstobtained and referenced, following which, reflectance data from theDUV-NIR region is collected and referenced. Once both data sets arerecorded they are spliced together to form a single broad band spectrum.

The instrument is separated into two environmentally controlledchambers, the instrument chamber 3202 and the sample chamber 3204. Theinstrument chamber 3202 houses most of the system optics and is notopened to the atmosphere on a regular basis. The sample chamber 3204houses the sample 3206 and a reference optic mirror M-5 and is openedregularly to facilitate changing samples.

In operation the VUV data is first obtained by switching flip-in sourcemirror FM-1 into the “out” position so as to allow light from the VUVsource 3201 to be collected, collimated and redirected towards beamsplitter element BS by focusing mirror M-1. Light striking the beamsplitter is divided into two components, the sample beam 3210 and thereference beam 3212, using a balanced Michelson interferometerarrangement. The sample beam is reflected from the beam splitter BS andtravels through shutter S-1. Shutter S-2 is closed during this time. Thesample beam continues on through compensator plate CP and is redirectedand focused into the sample chamber through window W-1 via focusingmirror M-2. The compensator plate is included to eliminate the phasedifference that would occur between the sample and reference pathsresulting from the fact that light traveling in the sample channelpasses through the beam splitter substrate but once, while lighttraveling in the reference channel passes through the beam splittersubstrate three times due to the nature of operation of a beam splitter.Hence, the compensator plate is constructed of the same material and isof the same thickness as the beam splitter. This ensures that lighttraveling through the sample channel also passes through the same totalthickness of beam splitter substrate material. Window W-1 is constructedof a material that is sufficiently transparent to VUV wavelengths so asto maintain high optical throughput in the system as described above.

Light entering the sample chamber 3204 strikes the sample 3206 and isreflected back through W-1 where it is collected, collimated andredirected by mirror M-2. Light from mirror M-2 travels throughcompensator plate CP, shutter S-1 and beam splitter BS, where it passesunhampered by flip-in detector mirror FM-2 (switched to the “out”position at the same time as FM-1), where it is redirected and focusedonto the entrance slit of the VUV spectrometer 3214 by focusing mirrorM-3. At this point light from the sample beam is dispersed by the VUVspectrometer and acquired by its associated detector.

Following collection of the sample beam 3210, the reference beam 3212 ismeasured. This is accomplished by closing shutter S-1 and openingshutter S-2. This enables the reference beam 3212 to travel through beamsplitter BS and shutter S-2, wherein it is redirected and focused intothe sample chamber through window W-2 via focusing mirror M-4. WindowW-2 is also constructed of a material that is sufficiently transparentto VUV wavelengths so as to maintain high optical throughput in thesystem as described above.

Once inside the sample chamber 3204, light is reflected from the surfaceof plane reference mirror M-5 and is reflected back towards mirror M-4where it is collected, collimated and redirected towards beam splitterBS. Light is then reflected by beam splitter BS towards mirror M-3 whereit is redirected and focused onto the entrance slit of the VUVspectrometer 3214.

Once both the sample and reference beams are collected a processor (notshown) can be used to calculate the referenced VUV reflectance spectrum.

Following measurement of the VUV data set, the DUV-NIR data is obtainedby switching both the source and detector flip-in mirrors, FM-1 and FM-2respectively, into the “in” position. As a result, light from the VUVsource 3201 is blocked and light from the DUV-NIR source 3203 is allowedto pass through window W-3, after it is collected, collimated andredirected by focusing mirror M-6. Similarly, switching flip-in mirrorFM-2 into the “in” position directs light from the sample beam 3210(when shutter S-1 is open and shutter S-2 is closed) and reference beam3212 (when shutter S-2 is open and shutter S-1 is closed) through windowW-4 onto mirror M-7 which focuses the light onto the entrance slit ofthe DUV-NIR spectrometer 3216 where it is dispersed and collected by itsdetector. Suitable DUV-NIR spectrometers and detectors are commonplacein today's market. A particularly well-matched combination ismanufactured by Jobin Yvon of France. The VS-70 combines a highlyefficient (f/2) optical design that does not employ turning mirrors.This instrument has a small physical footprint, incorporates an ordersorting filter and can be used with either a linear CCD or PDA detector.

The flip-in mirrors utilized into the system are designed such that theyare capable of switching position quickly and in a repeatable fashion inorder to minimize losses in optical throughput associated with errors inbeam directionality. A particularly well suited motorized flip-in mirroris manufactured by New Focus of the United States. In a slightlymodified embodiment, these mirrors could be replaced altogether by beamsplitter/shutters pairs; however this would be accompanied by anundesirable loss in VUV signal strength.

Once both the sample and reference beams are obtained the processor isused to calculate the referenced DUV-NIR reflectance spectrum. In thismanner, referenced reflectance data is serially obtained in the VUV andDUV-NIR spectral regions. It is noted that both the VUV and DUV-NIRspectrometers need be equipped with necessary sorting filters to avoidcomplications due to higher order diffraction components.

As vacuum compatible components are typically more complicated to designand expensive to manufacture than their standard counterparts, itfollows that system elements not critical to VUV operation be mountedoutside the controlled environment. Hence, the DUV-NIR source 3203 andspectrometer/detector 3216 are mounted outside the controlledenvironment. Such an arrangement is not required however.

A flowchart 3300 summarizing the serial collection process associatedwith the operation of this embodiment of the invention is presented inFIG. 33. More particularly, as shown in step 3302, the system firstenables collection of a VUV spectral region by switching flip-in source(FM-1) and detector (FM-2) mirrors into the “out” position. Then in step3304, the system starts VUV sample channel data acquisition by openingshutter S-1. Further, in step 3306, VUV sample channel data acquisitionis stopped by closing shutter S-1. Then in step 3308, VUV referencechannel data acquisition is started by opening shutter S-2. Next in step3310, VUV reference channel data acquisition is stopped by closingshutter S-2. Further in step 3312, the VUV reflectance spectrum iscalculated. Then, in step 3314, collection of the DUV-NIR spectralregion is enabled by switching flip-in source (FM-1) and detector (FM-2)mirrors into “in” position. Next, in step 3316 the DUV-NIR samplechannel data acquisition is started by opening shutter S-1. Then in step3318 DUV-NIR sample channel data acquisition is stopped by closingshutter S-1. Then in step 3320 the DUV-NIR reference channel dataacquisition is started by opening shutter S-2. Next in step 3322,DUV-NIR reference channel data acquisition is stopped by closing shutterS-2. Further in step 3324, DUV-NIR reflectance spectrum is calculated.Then in step 3326, data from VUV and DUV-NIR spectral regions is splicedtogether to obtain single broad band reflectance spectrum.

There are many benefits afforded by this embodiment of the invention.For example, the system has been optimized for efficient and accurateVUV performance. Among other things this requires that the environmentcontaining the optical path traversed by the VUV photons be controlledsuch that concentrations of absorbing species like oxygen and moistureare maintained at sufficiently low levels so as to permit adequateoptical throughput. This may be achieved in a variety of ways asdescribed in more detail above. Such techniques include purging theenvironment with a non-absorbing gas and/or through evacuation via avacuum system, depending on the level of system performance required.

During acquisition of the VUV data, the flip-in source and detectormirrors are switched to the “out” position and therefore do notcontribute any mechanical uncertainty to the measurement. In fact, thereare no moving optical elements involved in the acquisition of the VUVdata (aside from the shutters). This is advantageous for multiplereasons. Firstly, short wavelength VUV measurements are typically morechallenging to perform than those in other wavelength regions due to thelow photon flux available and general shortage of efficient optics andcoatings that can be employed. Secondly, characterization of ultra-thin(<100 Å) films relies heavily on accurate intensity or amplitudeinformation, since reflectance spectra from such films do not generallyexhibit distinct spectral features related to interference effects as dospectra from their thicker film counterparts.

Another benefit afforded by this embodiment is that it provides a fastand automated means of referencing collected data sets such that highlyrepeatable results are achieved. This capability acts to reduce oraltogether remove errors introduced by changes in the optical throughputof the system. At longer wavelengths such changes are typically drivenby variations in source output, while in the VUV changes in theconcentrations of absorbing species in the environment of the opticalpath are expected to dominate.

A further benefit afforded by this embodiment of the invention relatesto use of the single optical delivery/collection module. This commonmodule acts to facilitate the collection of data from the same locationon the sample using the same spot size and orientation regardless of thespectral region investigated. To this end, the DUV-NIR source andspectrometer are selected such that they maintain substantially similarlight gathering/delivery characteristics to those of the VUV source andspectrometer. This aspect of the system is particularly important insituations where patterned samples are under study.

Additionally, the single optical module simplifies alignment of theinstrument during integration and manufacturing, particularly withrespect to complications arising from auto-focusing routines.

A further benefit afforded by the invention arises from its serialmethod of operation. Stray light generated through scattering processesmay be problematic since it cannot be simply referenced out and can thuslead to non-linear system response and inaccuracies in the recordedreflectance data. By collecting and referencing data from each of thewavelength regions sequentially, the intensity of scattered photonsrecorded at the detector can be significantly reduced. This followssince only light from one source propagates through the system at anygiven time. Hence, light from other spectral regions cannot scatter andlead to spurious signals recorded at the detector. This is particularlyadvantageous when working in the VUV wavelength region, as scatteringmechanisms play a much larger role than at longer wavelengths.

The broad band system and techniques described above can be readilyextended to encompass other spectral regions simply through the additionof alternate sources, spectrometers and detectors. FIG. 34 presents analternate broad band system 3400 in one embodiment of the inventionoptimized for operation in a first spectral region and designed toperform well in second and third spectral regions. For example, inaddition to sources 3201 and 3203 such as shown in FIG. 32, a thirdsource 3302 may be utilized. In one embodiment, the source 3201 may be aVUV source, the source 3203 may be a DUV source and the source 3302 maybe a NIR source. Corresponding VUV spectrometer 3214, DUV spectrometer3216 and NIR spectrometer 3304 may be utilized with the respectiveassociated source. As before, sets of source and detector flip-inmirrors are used to deliver light from alternate sources and toalternate spectrometers and detectors. In this embodiment first spectralregion data is collected with all flip-in mirrors (FM-1, FM-2, FM-3, andFM-4) in the “out” position, second spectral region data is collectedutilizing source 3203 with only flip-in mirrors FM-1 and FM-2 switchedto the “in” position and third spectral region data is collectedutilizing source 3302 with only FM-3 and FM-4 switched to the “in”position.

A flowchart 3500 of the serial measurement process for this embodimentis presented in FIG. 35. More particularly, as shown in step 3502,collection of the first spectral region is enabled by switching allflip-in source and detector mirrors into the “out” position. Then instep 3504, a first sample channel data acquisition is started by openingshutter S-1. Next, in step 3506 the first sample channel dataacquisition is stopped by closing shutter S-1. Further in step 3508, thefirst reference channel data acquisition is started by opening shutterS-2. Then in step 3510, the first reference channel data acquisition isstopped by closing shutter S-2. Then in step 3512, the first spectralregion reflectance spectrum is calculated. Further in step 3514,collection of the second spectral region is enabled by switching flip-insource (FM-1) and detector (FM-2) mirrors into the “in” position. Nextin step 3516, the second spectral region sample channel data acquisitionis started by opening shutter S-1. Next in step 3518, the secondspectral region sample channel data acquisition is stopped by closingshutter S-1. Then in step 3520, the second spectral region referencechannel data acquisition is started by opening shutter S-2. Next in step3522, the second spectral region reference channel data acquisition isstopped by closing shutter S-2. Further in step 3524, the secondspectral region reflectance spectrum is calculated. Then in step 3526,collection of the third spectral region is enabled by switching flip-insource (FM-1) and detector (FM-2) mirrors into the “out” position andflip-in source (FM-3) and detector (FM-4) mirrors into the “in”position. Then in step 3528, the third spectral region sample channeldata acquisition is started by opening shutter S-1. Next in step 3530,the third spectral region sample channel data acquisition is stopped byclosing shutter S-1. Next in step 3532, the third spectral regionreference channel data acquisition is started by opening shutter S-2.Further in step 3534, the third spectral region reference channel dataacquisition is stopped by closing shutter S-2. Then in step 3536, thethird spectral region reflectance spectrum is calculated. Next in step3538, data from the first, second and third spectral regions is splicedtogether to obtain a single broad band reflectance spectrum.

An alternate broad band system 3600, in one embodiment of the invention,is presented in FIG. 36 wherein the selection of sources 3201 and 3203and spectrometers 3214 and 3216 is accomplished through the rotation offocusing optics RM-1 and RM-2, rather than through use of flip-inmirrors. In this embodiment RM-1 and RM-2 are off-axis parabolic mirrorswith a turning angle of 90°. Hence, RM-1 can be rotated about theoptical axis defined by the line joining RM-1 and beam splitter BS inorder to collect light from either the VUV source 3201 or DUV-NIR source3203. Similarly, focusing mirror RM-2 can be rotated about the axisdefined by the line between RM-2 and BS in order to focus light onto theentrance slit of either the VUV spectrometer 3214 or DUV-NIRspectrometer 3216.

This arrangement utilizes fewer optical components than that of theembodiment of FIG. 32 and hence, may render a smaller instrumentalfootprint. A potential drawback to this approach is that it doesintroduce some degree of mechanical uncertainty into the VUV measurementprocess due to the rotation of focusing optics RM-1 and RM-2.

An alternate embodiment of the invention is presented in FIG. 37 withregard to system 3700 wherein the balanced interferometer employed inthe referencing channel 3212 is of the Mach-Zehnder type rather than theMichelson configuration depicted in the embodiments of FIG. 32, FIG. 34and FIG. 36. This embodiment requires additional optical elements butprovides greater flexibility with respect to the angular delivery andcollection of light to and from the surface of sample 3206.

In operation, light from the first source 3201 is collected, collimatedand re-directed by focusing mirror M-1 towards beam splitter BS-1 whereit is divided into sample beam 3210 and reference beam 3212 components.The sample beam 3210 is enabled when shutter S-1 is open and shutter S-2is closed. In this state light reflected from beam splitter BS-1 iscollected and focused onto the sample through window W-1 by focusingmirror M-2. Light reflected from the sample 3206 exits the samplechamber 3204 via window W-1 and is collected, collimated and redirectedby focusing mirror M-3 towards plane mirror M-4. Light leaving mirrorM-4 travels through the second beam splitter BS-2 and is collected andfocused onto the entrance slit of the first spectral region spectrometer3214 by focusing mirror M-5. At this point light from the sample beam3210 is dispersed by the spectrometer and acquired by the detector.

Following collection of the first spectral region sample beam, the firstspectral region reference beam is measured. This is accomplished byclosing shutter S-1 and opening shutter S-2. This allows the referencebeam 3212 to travel through beam splitter BS-1, wherein it is redirectedand focused into the sample chamber 3204 through window W-2 via focusingmirror M-6. Once inside the sample chamber 3204 light is reflected fromthe surface of plane reference mirror M-7 and is collected, collimatedand redirected towards plane mirror M-9 by focusing mirror M-8. Thislight is reflected by beam splitter BS-2, and is redirected and focusedonto the entrance slit of the first spectral region spectrometer 3214 byfocusing mirror M-5. Once both the sample and reference beams areobtained the referenced reflectance data for the first spectral regionis calculated using a processor (not shown).

Data from the second and third spectral regions are again collectedusing sets of source and detector flip-in mirrors to deliver light fromalternate sources and to alternate spectrometers and their associateddetectors. Specifically, second spectral region data is collected whenonly flip-in mirrors FM-1 and FM-2 are switched to the “in” position andthird spectral region data is collected when only FM-3 and FM-4 areswitched to the “in” position.

Yet another embodiment of the invention is presented as system 3800 inFIG. 38. This dual spectral region configuration also incorporates aMach-Zehnder interferometer referencing system but does not require theuse of flip-in mirrors to select between spectral regions. Instead, twoadditional source shutters (S-1 and S-4) have been added to the systemto accomplish this task. When measurements of the first spectral regionare performed shutter S-1 is open and shutter S-4 is closed. Conversely,when second spectral region measurements are performed shutter S-1 isclosed and S-4 is open.

As this embodiment does not utilize flip-in mirrors it follows thatsystem repeatability could to some extent be improved relative to thepreviously described embodiments since mechanical positioning errorsassociated with the flip-in mirrors have been removed.

A flowchart 3900 of the serial measurement process for the embodiment ofFIG. 38 is presented in FIG. 39. More particularly, as shown in step3902, collection of the first spectral region data is enabled by openingfirst source shutter S-1. Then in step 3904, the first spectral regionsample channel data acquisition is started by opening shutter S-2. Nextin step 3906, the first spectral region sample channel data acquisitionis stopped by closing shutter S-2. Then in step 3908, the first spectralregion reference channel data acquisition is started by opening shutterS-3. Further in step 3910, the first spectral region reference channeldata acquisition is stopped by closing shutter S-3. Then in step 3912,the first spectral region reflectance spectrum is calculated. Next instep 3914, collection of the second spectral region is enabled byclosing first source shutter S-1 and opening second source shutter S-4.Then in step 3916, the second spectral sample channel data acquisitionis started by opening shutter S-2. Next in step 3918, the secondspectral sample channel data acquisition is stopped by closing shutterS-2. Further in step 3920, the second spectral reference channel dataacquisition is started by opening shutter S-3. Then in step 3922, thesecond spectral reference channel data acquisition is stopped by closingshutter S-3. Next in step 3924, the second spectral reflectance spectrumis calculated. Then in step 3926, data from the first and secondspectral regions is spliced together to obtain a single broad bandreflectance spectrum.

Thus, as described above broad band systems are provided that may beoptimized for operation in a first spectral region and capable ofperforming well in at least one other spectral region. Common deliveryand collection optics in an optical module enable measurements inseparate spectral regions to be collected using similar spot properties.For example, similar spot sizes for collection from the sample may beobtained. Further the orientation of the collection spot may besubstantially similar between the different spectral regions.Furthermore, the systems and techniques described allow for a serialdata collection approach whereby data from separate spectral regions iscollected sequentially to avoid stray light complications. The systemsmay be designed such that no moving optical elements (apart fromshutters) are involved in the collection of data from the first spectralregion. Additionally, the systems may incorporated an optical modulewhich presents selectable sources and detectors optimized for separatespectral regions. The optical module may also provide a mechanism forquickly referencing measured data so as to ensure that highly repeatableresults are achieved.

The broad band systems and techniques described above accordinglyprovide a metrology system that allows for the accurate collection ofoptical metrology data from a sample over multiple spectral ranges. Byhaving a optical data for a wide range of wavelengths, fittingalgorithms employed by a user of the instrument may achieve fasterconvergence and more accurate results by taking full advantage of thehigher level of constraint afforded by a data set comprised of two ormore spectral regions.

When the optical data is collected for multiple spectral regions asdescribed above, the data may be combined in a computer, processor orthe like to form a continuous set of data that may be analyzed. The datamay be combined in a wide range of manners and, ideally, the data fromeach spectrometer would match at the point where the spectral regionsjoin. For example, predetermined wavelengths may be selected todetermine which spectrometer will be utilized to collect data forparticular wavelengths. For example, for wavelengths below 190 nm datamay be taken solely from the VUV spectrometer and for wavelengthsgreater than 190 the data may be taken from a DUV-NIR spectrometer.However, such an approach may cause discontinuities in the collecteddata at the wavelength crossover point if the results vary from eachspectrometer at the crossover point. Such variations may complicatefitting algorithms and the processing of the data. In another approach,the wavelength for which data is collected from each spectrometer mayoverlap for some determined range, for example 20 nm. In this overlapregion, the data for each wavelength may be calculated as an averagefrom each spectrometer. In yet another alternative graded averages orbest fit algorithms could be applied to join the data. Any othersuitable approach that combines the data from each spectral region mayalso be utilized.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as presentlypreferred embodiments. Equivalent elements may be substituted for thoseillustrated and described herein and certain features of the inventionmay be utilized independently of the use of other features, all as wouldbe apparent to one skilled in the art after having the benefit of thisdescription of the invention.

1. (canceled)
 2. A method for analyzing a scattering or diffracting structure, comprising: providing a below deep ultra-violet (DUV) wavelength reflectometer configured for normal incidence operation and having a light source that provides at least below DUV wavelength light; obtaining reflectance data from the scattering or diffracting structure, including reflectance data for wavelengths below DUV wavelengths; and utilizing the reflectance data to obtain dimension information of the scattering or diffracting structure.
 3. The method of claim 2, further comprising utilizing referencing to account for system and environmental changes to adjust reflectance data obtained through use of the reflectometer.
 4. The method of claim 2, wherein the reflectometer utilizes a non-polarizing optical path such that a polarization independent measurement may be obtained.
 5. The method of claim 2, wherein the below DUV wavelength light is non-polarized at a point in an optical path of the reflectometer at which the scattering or diffracting structure is located.
 6. The method of claim 2, wherein the dimension information comprises at least one of line width, sidewall angle, line height, trench depth, trench width and film thickness.
 7. The method of claim 2, wherein scatterometry techniques are utilize to determine the dimension information.
 8. The method of claim 7, wherein rigorous coupled wave analysis techniques are utilized to determine the dimension information.
 9. A method for analyzing a scattering or diffracting structure, comprising: providing a below deep ultra-violet (DUV) wavelength referencing reflectometer configured for normal incidence operation and having a non-polarizing optical system that provides at least below deep ultra-violet wavelength light, and utilizing scatterometry techniques for obtaining dimensional information of the scattering or diffracting structure.
 10. The method of claim 9, further comprising utilizing referencing to account for system and environmental changes to adjust reflectance data obtained through use of the reflectometer.
 11. The method of claim 10, wherein the dimension information comprises at least one of line width, sidewall angle, line height, trench depth, trench width and film thickness.
 12. The method of claim 9, wherein the dimension information comprises at least one of line width, sidewall angle, line height, trench depth, trench width and film thickness.
 13. The method of claim 9, wherein rigorous coupled wave analysis techniques are utilized to determine the dimension information.
 14. A scatterometry method to determine dimensional information of a patterned structure, comprising: providing a reflectometer that utilizes as least in part below deep ultra-violet (DUV) wavelength light; obtaining reflectance data from the scattering or diffracting structure, including reflectance data for at least some wavelengths below DUV wavelengths; and utilizing scatterometry techniques to analyze the reflectance data to obtain dimension information of the pattern structure.
 15. The method of claim 14, wherein the reflectometer utilizes a non-polarizing optical path such that a polarization independent measurement may be obtained.
 16. The method of claim 14, wherein the below DUV wavelength light is non-polarized at a point in an optical path of the reflectometer at which the patterned structure is located.
 17. The method of claim 14, wherein the dimension information comprises at least one of line width, sidewall angle, line height, trench depth, trench width and film thickness.
 18. The method of claim 14, wherein the scatterometry techniques include rigorous coupled wave analysis techniques. 